Photoluminescent compounds

ABSTRACT

Materials comprising an A/M/X compound are provided. An A/M/X compound is a compound comprising one or more A moieties, one or more M atoms and one or more X atoms, where the A moieties are selected from organic cations and elements from Group 1 of the periodic table, the M atoms are selected from elements from Group 14 of the periodic table, and the X atoms are selected from elements from Group 17 of the periodic table. The materials include two-phase materials.

The present application is a divisional of U.S. patent application Ser.No. 13/771,309 filed on Feb. 20, 2013, now issued U.S. Pat. No.9,181,475, the entire contents of which are hereby incorporated byreference; which claims priority from U.S. provisional patentapplication Ser. No. 61/601,262, filed on Feb. 21, 2012, and from U.S.provisional patent application Ser. No. 61/601,219, filed on Feb. 21,2012, both of which are incorporated herein by reference.

BACKGROUND

Some metal halide compounds such as CsSnI₃ have been reported to exhibitstrong photoluminescence (PL) at RT.

Thin films of CsSnI₃, CsSnI₂Cl and CsSnICl₂ have been prepared by thethermal and e-beam evaporation methods. However, such high vacuumtechniques are expensive, inadequate for controlling compositions andhomogeneity and can suffer from reproducibility.

SUMMARY

Material comprising an A/M/X compound are provided. Also provided aresolution-based methods of making film comprising the A/M/X compounds andphotoluminescent light sources incorporating the materials. An A/M/Xcompound is a compound comprising one or more A moieties, one or more Matoms and one or more X atoms, where the A moieties are selected fromorganic cations and elements from Group 1 of the periodic table, the Matoms are selected from elements from Group 14 of the periodic table,and the X atoms are selected from elements from Group 17 of the periodictable. In the A/M/X compounds, M may represent two or more different Matoms (i.e., different M elements) and/or X may represent two or moredifferent X atoms (e.g., different halogen elements).

The compounds are electrically conductive and photoluminescent,characterized by photoluminescence wavelengths in the range of about 400nm to about 2000 nm at room temperature (23° C.). This includescompounds having photoluminescent wavelengths in the range from about700 nm to about 2000 nm and compounds having photoluminescencewavelengths in the range of about 800 nm to about 1200 nm at roomtemperature (23° C.).

One embodiment of the material comprises a compound having a formulaselected from AMX₃, A₂MX₄, AM₂X₅ or A₂MX₆, where the A is selected fromelements from Group 1 of the periodic table, the M is selected fromelements from Group 14 of the periodic table, and X is selected fromelements from Group 17 of the periodic table, and either metal oxides ofthe formula M′O_(y′), where y′=1, 1.5 or 2, or metal halides of theformulas A₂M′X₆ or A₃M′X₆, where M′ is selected from elements of Groups2-4 or elements of Groups 12-15 of the periodic table. The metal oxideor metal halide compounds can form a second phase in the material, suchthat the material comprises a first phase of the A/M/X compounds andsecond phase of the metal oxide or metal halide compounds. In somemembers of this embodiment, the compound has the formula AMX₃, such thatthe material has the formula (AMX₃)_((1−x))(M′O_(y)′)_(x). This includescompounds wherein AMX₃ is CsSnI₃. This also includes compounds havingthe formula (AMX₃)_((1−x))(AM′_(0.5)F₃)_(x), where 0.1≤x≤0.5 and M′ isselected from Si, Ge, Sn, Ti, Zr, and Hf. This also includes compoundshaving the formula Cs(SnI₃)_((1−x))(SnO)_(x), where 0.1≤x≤0.5. In othermembers of this embodiment, the compound has the formula A₂MX₄, suchthat the material has the formula (A₂MX₄)_((1−x))(M′O_(y)′)_(x). Thisincludes compounds wherein (A₂MX₄) is Cs₂SnI₄. The metal oxides includeoxides of metals selected from Group 3, Group 4, Group 12, Group 13,Group 14, and Group 15 of the periodic table. For example, TiO₂ is asuitable metal oxide. In the formulas, 0.01<x<0.99, or 0.1≤x≤0.6, or0.1≤x≤0.5.

In some members of this embodiment, the AMX₃ compound comprises two Xelements, such that the compound has the formula AMX_((3−x))X′_(x),where X and X′ are different halogen atoms and 0.01<x<0.99. In some suchmembers, the material is a two-phase material comprising a first phaseof AMX_((3−x))X′_(x) and a second phase comprising a metal halide or ametal oxide.

Specific members of this embodiment include materials selected from thegroup consisting of (CsSnI₃)_(1−x)(M′O)_(x), where M′=Si, Ge, Sn, orPb); (CsSnI₃)_(1−x)(M′O_(1.5))_(x), where M′=B, Al, Ga, In, Sc, or Y;(CsSnI₃)_(1−x)(M′O₂)_(x), where M′=Si, Ge, Sn, or Pb;(Cs₂SnI₄)_(1−x)(M′O)_(x), where M′=Si, Ge, Sn, or Pb;(Cs₂SnI₄)_(1−x)(M′O_(1.5))_(x), where M′=B, Al, Ga, In, Sc, or Y;(Cs₂SnI₄)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb;Cs(SnI₃)_(1−x)(M′O)_(x), M′=Si, Ge, Sn, or Pb;Cs(SnI₃)_(1−x)(M′O_(1.5))_(x), M′=B, Al, Ga, In, Sc, or Y;Cs(SnI₃)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb;Cs₂(SnI₄)_(1−x)(M′O)_(x), M′=Si, Ge, Sn, or Pb;Cs₂(SnI₄)_(1−x)(M′O_(1.5))_(x), M′=B, Al, Ga, In, Sc, or Y; andCs₂(SnI₄)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb, where 0.01<x<0.99.

In some members of this embodiment, in which the material comprises ametal halide, the metal halide is a metal fluoride. In some members, themetal halide comprises two or more M′ elements. In the embodiments inwhich the material comprises a metal halide, the compound may comprise,for example, CsSnI₃ or Cs₂SnI₄.

Specific members of this embodiment include materials selected from thegroup consisting of (CsSnI₃)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x), M′=Al, Ga,In, Sc, or Y; (CsSnI₃)_(1−x)(CsM′_(0.5)F₃)_(x), M′=Si, Ge, Sn, Ti, Zr,or Hf; (Cs₂SnI₄)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x), M′=Al, Ga, In, Sc, or Y;(Cs₂SnI₄)_(1−x)(CsM′_(0.5)F₃)_(x), M′=Si, Ge, Sn, Ti, Zr, or Hf;Cs(SnI₃)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x), M′=Al, Ga, In, Sc, or Y;Cs(SnI₃)_(1−x)(CsM′_(0.5)F₃)_(x), M′=Si, Ge, Sn, Ti, Zr, or Hf; where0.01<x<0.99.

Another embodiment of the materials comprises compounds having a formulaselected from AMX₃, A₂MX₄, AM₂X₅ or A₂MX₆, where the A is a monovalentorganic ammonium cation, M represents one or more elements selected fromelements from Group 14 of the periodic table, and X represents one ormore halogen elements selected from elements from Group 17 of theperiodic table, wherein the compound comprises more than one M element,more than one X element, or more than one of both. For example, incompounds comprising two M elements, those elements can be selected fromSn, Pb, Ge and Si. This includes members of this embodiment in which atleast one of the M elements comprises Ge. Further included are compoundscomprising Sn_((1−x))Pb_(x), where 0.01<x<0.99. One such compound isCH₃NH₃Sn_((1−x))Pb_(x)I₃.

Specific examples of compounds which include more than one halogenelement are those selected from the group consisting of CH₃NH₃SnF₂I,CH₃NH₃SnCl₂I, CH₃NH₃SnBr₂I, CH₃NH₃SnI₂F, CH₃NH₃SnI₂Cl, CH₃NH₃SnI₂Br,HC(NH₂)₂SnF₂I, HC(NH₂)₂SnCl₂I, HC(NH₂)₂SnBr₂I, HC(NH₂)₂SnI₂Cl,HC(NH₂)₂SnI₂F and HC(NH₂)₂SnI₂Br.

Examples of monovalent organic ammonium cations include methyl ammoniumions, formamidinium ion, methylformamidinium ions and guanidinium ions.

Other embodiments of the materials comprise compounds having a formulaselected from AMX₃, A₂MX₄, AM₂X₅ or A₂MX₆, where A is amethylformamidinium ion or a guanidinium ion, M is selected fromelements from Group 14 of the periodic table, and X is a halogen atom.While still other embodiments of the materials comprise compounds havinga formula selected from AMX₃, A₂MX₄, AM₂X₅ or A₂MX₆, where A is anorganic ammonium ion, M is Ge and X is a halogen atom.

Photoluminescent light sources that incorporate the materials comprise aradiation source configured to generate radiation at an incidentwavelength; and one or more of the materials recited herein, configuredsuch that it is irradiated by radiation from the radiation source whenthe source is on. In the photoluminescent sources, the radiation and thematerial are characterized in that the material is able to absorb theradiation, whereby the material is induced to emit photoluminescence inthe wavelength range of 400 to 2000 nm. The photoluminescent sourcesinclude infrared and near-infrared sources, in which the materialphotoluminesces in the wavelength range of 700 to 2000 nm. Optionalcomponents of the photoluminescent sources include more filtersconfigured to which block emitted photoluminescence radiation having anundesired wavelength, while selectively transmitting emittedphotoluminescence at other wavelengths. The source may also optionallyinclude a photoluminescence detector configured such that thephotoluminescence from the material impinges on the detector when thesource is in operation.

Other devices into which the materials can be incorporated include lightemitting diodes, sensors, and photovoltaic cells.

An embodiment of a solution-based film deposition process for A/M/Xcompounds comprises the steps of: forming a homogeneous solution of anA/M/X compound in a polar organic solvent; deposing the solution onto asubstrate; and drying the deposited solution, whereby a film comprisingthe A/M/X compound is formed. Suitable polar organic solvents includeacetonitrile, DMF, ethanol and mixtures thereof. Depositing the solutiononto a substrate can be accomplished, for example, by drop-coating,spin-coating or spraying the solution onto the substrate. Drying thedeposited solution can be carried out by heating the deposited solution.For example, the solution can be heated to a temperature of at least 50°C. Suitable substrates include silicon, glass, ceramic, metal foil andplastic substrates. The examples below describe the formation of a filmcomprising CsSnI₃ via a solution deposition process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the PL emission vs. wavelength for CsSnI₃,CsSnO_(0.1)I_(2.8), CSSnO_(0.2)I_(2.6), CSSnO_(0.3)I_(2.4),CSSnO_(0.4)I_(2.2), CSSnO_(0.5)I_(2.0), CSSnO_(0.6)I_(1.8),CSSnO_(0.7)I_(1.6), CsSnO_(0.8)I_(1.4), CsSnO_(0.9)I₂, CsSnOI, and InP.

FIG. 2 shows the optical absorption spectra for different compositionsof Cs(SnI₃)_(1−x)(MO)_(x).

FIG. 3. PL emission vs. wavelength of Cs₂Sn₃OI₆, Cs₄Sn₃OI₈,Cs₂Sn₂B₂O₃I₆, Cs₄Sn₂B₂O₃I₈, Cs₂Sn₂Al₂O₃I₆, Cs₄Sn₂Al₂O₃I₈, CsSnI₃, andInP.

FIG. 4. Optical absorption spectra of Cs₂Sn₃OI₆, Cs₄Sn₃OI₈,Cs₂Sn₂B₂O₃I₆, Cs₄Sn₂B₂O₃I₈, Cs₂Sn₂Al₂O₃I₆, Cs₄Sn₂Al₂O₃I₈, and CsSnI₃.

FIG. 5A. PL emission vs. wavelength of Cs₂Sn₂SiF₆I₄, Cs₄Sn₂SiF₆I₆,Cs₆Sn₂SiF₆I₈, CsSnPF₆I₂, Cs₂SnPF₆I₃, Cs₃SnPF₆I₄, CsSnI₃, and InP. FIG.5B. PL emission vs. wavelength of the Cs₄Sn₂SiF₆I₆ that saturated thedetector, measured at lower integration time. FIG. 5C. PL emission vs.wavelength of the Cs₆Sn₂SiF₆I₈ phase that saturated the detector,measured at lower integration time.

FIG. 6. Optical absorption of Cs₂Sn₂SiF₆I₄, Cs₄Sn₂SiF₆I₆, Cs₆Sn₂SiF₆I₈,CsSnPF₆I₂, Cs₂SnPF₆I₃, Cs₃SnPF₆I₄, and CsSnI₃.

FIG. 7A. Optical absorption vs. energy of Type IIB materials on 785 nm(1.58 eV) excitation as measured using a Raman scattering setup. FIG.7B. PL emission vs. energy of Type IIB materials on 785 nm (1.58 eV)excitation as measured using a Raman scattering setup.

FIG. 8. PL emission vs. wavelength for CsSn(I_(3−x)X_(x)) compositions.

FIG. 9. Optical absorption spectra for CsSn(I_(3−x)X_(x)) compositions.

FIG. 10. PL emission vs. wavelength for CH₃NH₃Sn(I_(3−x)X_(x))compositions.

FIG. 11. Optical absorption for CH₃NH₃Sn(I_(3−x)X_(x)) compositions.

FIG. 12A. PL emission vs. wavelength for HC(NH₂)₂Sn(I_(3−x)X_(x))compositions showing a first emission wavelength range. FIG. 12B. PLemission vs. wavelength for HC(NH₂)₂Sn(I_(3−x)X_(x)) compositionsshowing a second emission wavelength range.

FIG. 13. Optical absorption spectra for HC(NH₂)₂Sn(I_(3−x)X_(x))compositions.

FIG. 14A. PL emission vs. wavelength for CH₃NH₃Pb(I_(3−x)X_(x)) phases,showing a first emission wavelength range. FIG. 14B. PL emission vs.wavelength for CH₃NH₃Pb(I_(3−x)X_(x)) phases, showing a second emissionwavelength range.

FIG. 15. Optical absorption spectra for CH₃NH₃Pb(I_(3−x)X_(x))compositions.

FIG. 16. PL emission vs. wavelength for germanium iodide phases.

FIG. 17. Optical absorption spectra for germanium iodide phases.

FIG. 18. PL emission vs. wavelength for CH₃NH₃Sn_(1−x)Pb_(x)I₃ fordifferent values of x.

FIG. 19. Optical absorption for CH₃NH₃Sn_(1−x)Pb_(x)I₃ for differentvalues of x.

FIG. 20. PL emission vs. wavelength for CH₃NH₃SnCl₂I andCH₃NH₃Sn_(0.9)Co_(0.1)Cl₂I.

FIG. 21. Scanning electron microscopy images of CsSnI₃ thin films,showing continuous, polycrystalline nature of the films.

FIG. 22A. Electronic absorption spectra of specimens 1-4 of Example 2highlighting the band gap variability on the basis of the syntheticapproach that was employed to prepare the sample. FIG. 22B. Electronicabsorption spectra of specimens 1-4 of Example 2 highlighting the bandgap variability on the basis of the synthetic approach that was employedto prepare the sample. FIG. 22C displays the spectra of specimens of 1-4prepared with the solution method.

FIG. 23A. Photoluminescence spectra of ASnI₃ as obtained from the opentube reaction. The data are normalized towards a single crystallinesample of InP which is used as a benchmark. The Cs analogues, preparedin the same manner are given for comparison purposes. FIG. 23B.Photoluminescence spectra of APbI₃ as obtained from the open tubereaction. The data are normalized towards a single crystalline sample ofInP which is used as a benchmark. The Cs analogues, prepared in the samemanner are given for comparison purposes.

FIG. 24A. Photoluminescence properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃ solidsolution as obtained from the mildly grinding the precursors (interfacereaction). The data are normalized towards single crystalline InP. FIG.24B. Photoluminescence properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃ solidsolution as obtained by annealing the same specimen at 200° C. for 2 hin a sealed tube. The data are normalized towards single crystallineInP.

FIG. 25A. Optical absorption properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃ solidsolution as obtained from. In the cases where there is ambiguity on theSn:Pb ratio the values were obtained from EDS analysis. FIG. 25B.Optical absorption properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃ solid solutionas obtained from open tube melt. In the cases where there is ambiguityon the Sn:Pb ratio the values were obtained from EDS analysis. FIG. 25C.Optical absorption properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃ solid solutionas obtained from sealed tube annealing. In the cases where there isambiguity on the Sn:Pb ratio the values were obtained from EDS analysis.FIG. 25D. Optical absorption properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃ solidsolution as obtained from solution reactions. In the cases where thereis ambiguity on the Sn:Pb ratio the values were obtained from EDSanalysis.

DETAILED DESCRIPTION

Provided are compounds having the formula A/M/X, wherein A is at leastone element from Group 1 of the periodic table or is an organic cation;M is at least one element from Group 14 of the periodic table; and X isat least one element from Group 17 of the periodic table. Examples oforganic cations, A, in A/M/X compounds include methylammonium (CH₃NH₃⁺), formamidinium (HC(NH)₂)₂ ⁺), methylformamidinium (H₃CC(NH)₂)₂ ⁺),guanidinium (C(NH)₂)₃ ⁺), N,N-alkyl bipyridines and acridine. In someembodiments, the compounds have a formula selected from AMX₃, A₂MX₄,AM₂X₅, A₂MX₆ and AMX₅.

The compounds can exhibit photoluminescence in the visible and nearinfrared region at room temperature (RT) (˜23° C.).

In some embodiments the A, M, and X atoms are mixed with othercongeners, for example, CsSnI₂Cl and Cs₂SnI₂Cl₂. In some embodiments,the materials comprise other main group elements, transition metalelements, lanthanide elements, actinide elements, or their compounds,such as Al₂O₃, SnO, SnF₂ and CsSn_(0.5)F₃. In some embodiments, provideenhanced emission intensity or emission wavelengths shifted to a desiredspectral range.

Also provided are solution-based thin film deposition processes formaking films of the A/M/X compounds. In the processes, thin films can bedeposited by spin-coating or spraying solutions on various large-areasubstrates such as Si, glasses and flexible plastic substrates at lowtemperatures (e.g., 120-150° C.). Some embodiments of thesolution-processed thin films can outperform single crystalline InP:Swafers in terms of their PL intensities.

Synthesis and Characterization of A/M/X Metal Halide Compounds that EmitPhotoluminescence in the Visible and Near Infrared Regions.

Classes of metal halide compounds, including those that exhibit emissionin the visible and infrared regions, can be categorized into five types.

The class of Type I materials comprises compounds of the formulas AMX₃,A₂M₂X₄, AM₂X₅ and A₂MX₆ and either metal oxides of the formula M′O_(y′),where y=1, 1.5 or 2, or metal halides of the formulas A₂MX₆ and A₃MX₆,where A is selected from at least one of elements of Group 1 or organiccations, M′ is selected from at least one of elements of Groups 2-4 orelements of Groups 12-15 and X is an element from Group 17.

The class of Type II includes compounds having organic countercations.This class has two sub-types: Type IIA and Type IIB. In Type IIAcompound, ‘A’ comprises monovalent organic cations, such asmethylammonium (CH₃NH₃ ⁺), formamidinium (HC(NH)₂)₂ ⁺),methylformamidinium (H₃CC(NH)₂)₂ ⁺) and guanidinium (C(NH)₂)₃ ⁺). InType JIB compounds, ‘A’ comprises bulky π-conjugated divalent organiccations, such as N,N-alkyl bipyridines and acridine. The lattercompounds typically exhibit emission by a red light excitation at 785nm.

The class of Type III includes compounds in which ‘X’ comprises morethan one halide atom. Such compounds include CsSnI₂Cl and CsSnICl₂.

The class of Type IV includes compounds comprising Ge(IV). Examples ofType IV materials are GeI₂, GeI₄, AGeI₅ and A₂GeI₆ (A=alkali metal ororganic cations). GeI₄ exhibits photoluminescence.

The class of Type V comprises any combination of compounds of Type I,II, III, and/or IV.

The Type I materials typically are black in color. Examples of Type Imaterials, comprising metal oxides, include a) (CsSnI₃)_(1−x)(M′O)_(x),M′=Si, Ge, Sn, or Pb; b) (CsSnI₃)_(1−x)(M′O_(1.5))_(x), M′=B, Al, Ga,In, Sc, or Y; c) (CsSnI₃)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb; d)(Cs₂SnI₄)_(1−x)(M′O)_(x), M′=Si, Ge, Sn, or Pb; e)(Cs₂SnI₄)_(1−x)(M′O_(1.5))_(x), M′=B, Al, Ga, In, Sc, or Y; f)(Cs₂SnI₄)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb; g)Cs(SnI₃)_(1−x)(M′O)_(x), M′=Si, Ge, Sn, or Pb; h)Cs(SnI₃)_(1−x)(M′O_(1.5))_(x), M′=B, Al, Ga, In, Sc, or Y; i)Cs(SnI₃)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb; j)Cs₂(SnI₄)_(1−x)(M′O)_(x), M′=Si, Ge, Sn, or Pb; k)Cs₂(SnI₄)_(1−x)(M′O_(1.5))_(x), M′=B, Al, Ga, In, Sc, or Y; l)Cs₂(SnI₄)_(1−x)(M′O₂)_(x), M′=Si, Ge, Sn, or Pb, where 0.01<x<0.99.Examples of specific molar ratios of starting materials that can be usedto obtain materials of Type I are shown in Tables a)-l). The amounts ofthe starting materials in the tables are in mmol.

TABLE a (CsSnI₃)_(1−x)(M′O)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, or Pbx CsI SnI₂ PbO Composition 0 1.0 1.0 0 CsSnI₃ 0.1 0.90 0.90 0.10Cs_(0.9)Sn_(0.9)Pb_(0.1)I_(2.7)O_(0.1) 0.2 0.80 0.80 0.20Cs_(0.8)Sn_(0.8)Pb_(0.2)I_(2.4)O_(0.2) 0.3 0.70 0.70 0.30Cs_(0.7)Sn_(0.7)Pb_(0.3)I_(2.1)O_(0.3) 0.4 0.60 0.60 0.40Cs_(0.6)Sn_(0.6)Pb_(0.4)I_(1.8)O_(0.4) 0.5 0.50 0.50 0.50Cs_(0.5)Sn_(0.5)Pb_(0.5)I_(1.5)O_(0.5)

TABLE b (CsSnI₃)_(1−x)(M′O_(1.5))_(x): 0.01 < x < 0.99, M′ = B, Al, Ga,In, Sc, or Y x CsI SnI₂ Al₂O₃ Composition 0 1.0 1.0 0 CsSnI₃ 0.1 0.900.90 0.05 Cs_(0.9)Sn_(0.9)Al_(0.1)I_(2.7)O_(0.15) 0.2 0.80 0.80 0.10Cs_(0.8)Sn_(0.8)Al_(0.2)I_(2.4)O_(0.30) 0.3 0.70 0.70 0.15Cs_(0.7)Sn_(0.7)Al_(0.3)I_(2.1)O_(0.45) 0.4 0.60 0.60 0.20Cs_(0.6)Sn_(0.6)Al_(0.4)I_(1.8)O_(0.60) 0.5 0.50 0.50 0.25Cs_(0.5)Sn_(0.5)Al_(0.5)I_(1.5)O_(0.75)

TABLE c (CsSnI₃)_(1−x)(M′O₂)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, orPb x CsI SnI₂ SiO₂ Composition 0 1.0 1.0 0 CsSnI₃ 0.1 0.90 0.90 0.10Cs_(0.9)Sn_(0.9)Si_(0.1)I_(2.7)O_(0.20) 0.2 0.80 0.80 0.20Cs_(0.8)Sn_(0.8)Si_(0.2)I_(2.4)O_(0.40) 0.3 0.70 0.70 0.30Cs_(0.7)Sn_(0.7)Si_(0.3)I_(2.1)O_(0.60) 0.4 0.60 0.60 0.40Cs_(0.6)Sn_(0.6)Si_(0.4)I_(1.8)O_(0.80) 0.5 0.50 0.50 0.50Cs_(0.5)Sn_(0.5)Si_(0.5)I_(1.5)O_(1.00)

TABLE d (Cs₂SnI₄)_(1−x)(M′O)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, orPb x CsI SnI₂ PbO Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 1.90 0.90 0.10Cs_(1.9)Sn_(0.9)Pb_(0.1)I_(3.7)O_(0.1) 0.2 1.80 0.80 0.20Cs_(1.8)Sn_(0.8)Pb_(0.2)I_(3.4)O_(0.2) 0.3 1.70 0.70 0.30Cs_(1.7)Sn_(0.7)Pb_(0.3)I_(3.1)O_(0.3) 0.4 1.60 0.60 0.40Cs_(1.6)Sn_(0.6)Pb_(0.4)I_(2.8)O_(0.4) 0.5 1.50 0.50 0.50Cs_(1.5)Sn_(0.5)Pb_(0.5)I_(2.5)O_(0.5)

TABLE e (Cs₂SnI₄)_(1−x)(M′O_(1.5))_(x): 0.01 < x < 0.99, M′ = B, Al, Ga,In, Sc, or Y x CsI SnI₂ Al₂O₃ Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 1.900.90 0.05 Cs_(1.9)Sn_(0.9)Al_(0.1)I_(3.7)O_(0.15) 0.2 1.80 0.80 0.10Cs_(1.8)Sn_(0.8)Al_(0.2)I_(3.4)O_(0.30) 0.3 1.70 0.70 0.15Cs_(1.7)Sn_(0.7)Al_(0.3)I_(3.1)O_(0.45) 0.4 1.60 0.60 0.20Cs_(1.6)Sn_(0.6)Al_(0.4)I_(2.8)O_(0.60) 0.5 1.50 0.50 0.25Cs_(1.5)Sn_(0.5)Al_(0.5)I_(2.5)O_(0.75)

TABLE f (Cs₂SnI₄)_(1−x)(M′O₂)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, orPb x CsI SnI₂ SiO₂ Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 1.90 0.90 0.10Cs_(1.9)Sn_(0.9)Si_(0.1)I_(3.7)O_(0.20) 0.2 1.80 0.80 0.20Cs_(1.8)Sn_(0.8)Si_(0.2)I_(3.4)O_(0.40) 0.3 1.70 0.70 0.30Cs_(1.7)Sn_(0.7)Si_(0.3)I_(3.1)O_(0.60) 0.4 1.60 0.60 0.40Cs_(1.6)Sn_(0.6)Si_(0.4)I_(2.8)O_(0.80) 0.5 1.50 0.50 0.50Cs_(1.5)Sn_(0.5)Si_(0.5)I_(2.5)O_(1.00)

TABLE g Cs(SnI₃)_(1−x)(M′O)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, or Pbx CsI SnI₂ PbO Composition 0 1.0 1.0 0 CsSnI₃ 0.1 1.0 0.90 0.10CsSn_(0.9)Pb_(0.1)I_(2.8)O_(0.1) 0.2 1.0 0.80 0.20CsSn_(0.8)Pb_(0.2)I_(2.6)O_(0.2) 0.3 1.0 0.70 0.30CsSn_(0.7)Pb_(0.3)I_(2.4)O_(0.3) 0.4 1.0 0.60 0.40CsSn_(0.6)Pb_(0.4)I_(2.2)O_(0.4) 0.5 1.0 0.50 0.50CsSn_(0.5)Pb_(0.5)I_(2.0)O_(0.5)

TABLE h Cs(SnI₃)_(1−x)(M′O_(1.5))_(x): 0.01 < x < 0.99, M′ = B, Al, Ga,In, Sc, or Y x CsI SnI₂ Al₂O₃ Composition 0 1.0 1.0 0 CsSnI₃ 0.1 1.00.90 0.05 CsSn_(0.9)Al_(0.1)I_(2.8)O_(0.15) 0.2 1.0 0.80 0.10CsSn_(0.8)Al_(0.2)I_(2.6)O_(0.30) 0.3 1.0 0.70 0.15CsSn_(0.7)Al_(0.3)I_(2.4)O_(0.45) 0.4 1.0 0.60 0.20CsSn_(0.6)Al_(0.4)I_(2.2)O_(0.60) 0.5 1.0 0.50 0.25CsSn_(0.5)Al_(0.5)I_(2.0)O_(0.75)

TABLE i Cs(SnI₃)_(1−x)(M′O₂)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, orPb x CsI SnI₂ SiO₂ Composition 0 1.0 1.0 0 CsSnI₃ 0.1 1.0 0.90 0.10CsSn_(0.9)Si_(0.1)I_(2.8)O_(0.20) 0.2 1.0 0.80 0.20CsSn_(0.8)Si_(0.2)I_(2.6)O_(0.40) 0.3 1.0 0.70 0.30CsSn_(0.7)Si_(0.3)I_(2.4)O_(0.60) 0.4 1.0 0.60 0.40CsSn_(0.6)Si_(0.4)I_(2.2)O_(0.80) 0.5 1.0 0.50 0.50CsSn_(0.5)Si_(0.5)I_(2.0)O_(1.00)

TABLE j Cs₂(SnI₄)_(1−x)(M′O)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, orPb x CsI SnI₂ PbO Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 2.0 0.90 0.10Cs₂Sn_(0.9)Pb_(0.1)I_(3.8)O_(0.1) 0.2 2.0 0.80 0.20Cs₂Sn_(0.8)Pb_(0.2)I_(3.6)O_(0.2) 0.3 2.0 0.70 0.30Cs₂Sn_(0.7)Pb_(0.3)I_(3.4)O_(0.3) 0.4 2.0 0.60 0.40Cs₂Sn_(0.6)Pb_(0.4)I_(3.2)O_(0.4) 0.5 2.0 0.50 0.50Cs₂Sn_(0.5)Pb_(0.5)I_(3.0)O_(0.5)

TABLE k Cs₂(SnI₄)_(1−x)(M′O_(1.5))_(x): 0.01 < x < 0.99, M′ = B, Al, Ga,In, Sc, or Y x CsI SnI₂ Al₂O₃ Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 2.00.90 0.05 Cs₂Sn_(0.9)Al_(0.1)I_(3.8)O_(0.15) 0.2 2.0 0.80 0.10Cs₂Sn_(0.8)Al_(0.2)I_(3.6)O_(0.30) 0.3 2.0 0.70 0.15Cs₂Sn_(0.7)Al_(0.3)I_(3.4)O_(0.45) 0.4 2.0 0.60 0.20Cs₂Sn_(0.6)Al_(0.4)I_(3.2)O_(0.60) 0.5 2.0 0.50 0.25Cs₂Sn_(0.5)Al_(0.5)I_(3.0)O_(0.75)

TABLE l Cs₂(SnI₄)_(1−x)(M′O₂)_(x): 0.01 < x < 0.99, M′ = Si, Ge, Sn, orPb x CsI SnI₂ SiO₂ Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 2.0 0.90 0.10Cs₂Sn_(0.9)Si_(0.1)I_(3.8)O_(0.20) 0.2 2.0 0.80 0.20Cs₂Sn_(0.8)Si_(0.2)I_(3.6)O_(0.40) 0.3 2.0 0.70 0.30Cs₂Sn_(0.7)Si_(0.3)I_(3.4)O_(0.60) 0.4 2.0 0.60 0.40Cs₂Sn_(0.6)Si_(0.4)I_(3.2)O_(0.80) 0.5 2.0 0.50 0.50Cs₂Sn_(0.5)Si_(0.5)I_(3.0)O_(1.00)

Examples of Type I materials comprising metal halides are m)(CsSnI₃)_(1−x)(Cs_(1.5)M_(0.5)F₃)_(x), M′=Al, Ga, In, Sc, or Y; p)(CsSnI₃)_(1−x)(CsM′_(0.5)F₃)_(x), M′=Si, Ge, Sn, Ti, Zr, or Hf; q)(Cs₂SnI₄)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x), M′=Al, Ga, In, Sc, or Y; t)(Cs₂SnI₄)_(1−x)(CsM′_(0.5)F₃)_(x), M′=Si, Ge, Sn, Ti, Zr, or Hf; u)Cs(SnI₃)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x), M′=Al, Ga, In, Sc, or Y; x)Cs(SnI₃)_(1−x)(CsM′_(0.5)F₃)_(x), M′=Si, Ge, Sn, Ti, Zr, or Hf; where0.01<x<0.99. Examples of specific molar ratios of starting materialsthat can be used to obtain these Type I materials are shown in Tables m,p, q, t, u, and x. The amounts of the starting materials in the tablesare in mmol.

Tables n, r, and y list specific molar ratios of starting materials thatcan be used to obtain materials comprising compounds of the formula AMX₃or A₂MX₄ and metal halides of the formula MF₆. Tables o, s, and w listspecific molar ratios of starting materials that can be used to obtainmaterials comprising compounds of the formula AMX₃ or A₂MX₄ and metalhalides of the formula AMF₆. Tables y, z, and aa list specific molarratios for starting materials that can be used to obtain materialscomprising SnI₂ compounds and metal halides of the formula A₂MX₆, AMX₆,or MX₆.

Mixed Heteroatom Fluoride Compositions (1.0 Mmol Basis)

TABLE m (CsSnI₃)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ =Al, Ga, In, Sc, or Y x CsI SnI₂ AlF₃ CsF Composition 0 1.0 1.0 0 0CsSnI₃ 0.1 0.90 0.90 0.05 0.15 Cs_(1.05)Sn_(0.9)Al_(0.05)I_(2.7)F_(0.3)0.2 0.80 0.80 0.10 0.30 Cs_(1.10)Sn_(0.8)Al_(0.10)I_(2.4)F_(0.6) 0.30.70 0.70 0.15 0.45 Cs_(1.15)Sn_(0.7)Al_(0.15)I_(2.1)F_(0.9) 0.4 0.600.60 0.20 0.60 Cs_(1.20)Sn_(0.6)Al_(0.20)I_(1.8)F_(1.2) 0.5 0.50 0.500.25 0.75 Cs_(1.25)Sn_(0.5)Al_(0.25)I_(1.5)F_(1.5)

TABLE n (CsSnI₃)_(1−x)(Cs₀M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Al xCsI SnI₂ AlF₃ Composition 0 1.0 1.0 0 CsSnI₃ 0.1 0.90 0.90 0.10Cs_(0.9)Sn_(0.9)Al_(0.10)I_(2.7)F_(0.3) 0.2 0.80 0.80 0.20Cs_(0.8)Sn_(0.8)Al_(0.20)I_(2.4)F_(0.6) 0.3 0.70 0.70 0.30Cs_(0.7)Sn_(0.7)Al_(0.30)I_(2.1)F_(0.9) 0.4 0.60 0.60 0.40Cs_(0.6)Sn_(0.6)Al_(0.40)I_(1.8)F_(1.2) 0.5 0.50 0.50 0.50Cs_(0.5)Sn_(0.5)Al_(0.50)I_(1.5)F_(1.5)

TABLE o (CsSnI₃)_(1−x)(Cs_(0.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = P,As, Sb, V, Nb, or Ta x CsI SnI₂ CsPF₆ Composition 0 1.0 1.0 0 CsSnI₃ 0.10.90 0.90 0.05 Cs_(0.95)Sn_(0.9)P_(0.05)I_(2.7)F_(0.3) 0.2 0.80 0.800.10 Cs_(0.90)Sn_(0.8)P_(0.10)I_(2.4)F_(0.6) 0.3 0.70 0.70 0.15Cs_(0.85)Sn_(0.7)P_(0.15)I_(2.1)F_(0.9) 0.4 0.60 0.60 0.20Cs_(0.80)Sn_(0.6)P_(0.20)I_(1.8)F_(1.2) 0.5 0.50 0.50 0.25Cs_(0.75)Sn_(0.5)P_(0.25)I_(1.5)F_(1.5)

TABLE p (CsSnI₃)_(1−x)(CsM′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Si, Ge,Sn, Ti, Zr, or Hf x CsI SnI₂ Cs₂SiF₆ Composition 0 1.0 1.0 0 CsSnI₃ 0.10.90 0.90 0.05 CsSn_(0.9)Si_(0.05)I_(2.7)F_(0.3) 0.2 0.80 0.80 0.10CsSn_(0.8)Si_(0.10)I_(2.4)F_(0.6) 0.3 0.70 0.70 0.15CsSn_(0.7)Si_(0.15)I_(2.1)F_(0.9) 0.4 0.60 0.60 0.20CsSn_(0.6)Si_(0.20)I_(1.8)F_(1.2) 0.5 0.50 0.50 0.25CsSn_(0.5)Si_(0.25)I_(1.5)F_(1.5)

TABLE q (Cs₂SnI₄)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ =Al, Ga, In, Sc, or Y x CsI SnI₂ AlF₃ CsF Composition 0 2.0 1.0 0 0Cs₂SnI₄ 0.1 1.80 0.90 0.05 0.15 Cs_(1.95)Sn_(0.9)Al_(0.05)I_(3.7)F_(0.3)0.2 1.60 0.80 0.10 0.30 Cs_(1.90)Sn_(0.8)Al_(0.10)I_(3.4)F_(0.6) 0.31.40 0.70 0.15 0.45 Cs_(1.85)Sn_(0.7)Al_(0.15)I_(3.1)F_(0.9) 0.4 1.200.60 0.20 0.60 Cs_(1.80)Sn_(0.6)Al_(0.20)I_(2.8)F_(1.2) 0.5 1.00 0.500.25 0.75 Cs_(1.75)Sn_(0.5)Al_(0.25)I_(2.5)F_(1.5)

TABLE r (Cs₂SnI₄)_(1−x)(Cs₀M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Al xCsI SnI₂ AlF₃ Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.1 1.80 0.90 0.10Cs_(1.8)Sn_(0.9)Al_(0.10)I_(3.6)F_(0.3) 0.2 1.60 0.80 0.20Cs_(1.6)Sn_(0.8)Al_(0.20)I_(3.2)F_(0.6) 0.3 1.40 0.70 0.30Cs_(1.4)Sn_(0.7)Al_(0.30)I_(2.8)F_(0.9) 0.4 1.20 0.60 0.40Cs_(1.2)Sn_(0.6)Al_(0.40)I_(2.4)F_(1.2) 0.5 1.00 0.50 0.50Cs_(1.0)Sn_(0.5)Al_(0.50)I_(2.0)F_(1.5)

TABLE s (Cs₂SnI₄)_(1−x)(Cs_(0.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ =P, As, Sb, V, Nb, or Ta x CsI SnI₂ CsPF₆ Composition 0 2.0 1.0 0 Cs₂SnI₄0.1 1.80 0.90 0.05 Cs_(1.95)Sn_(0.9)P_(0.05)I_(3.7)F_(0.3) 0.2 1.60 0.800.10 Cs_(1.90)Sn_(0.8)P_(0.10)I_(3.4)F_(0.6) 0.3 1.40 0.70 0.15Cs_(1.85)Sn_(0.7)P_(0.15)I_(3.1)F_(0.9) 0.4 1.20 0.60 0.20Cs_(1.80)Sn_(0.6)P_(0.20)I_(2.8)F_(1.2) 0.5 1.00 0.50 0.25Cs_(1.75)Sn_(0.5)P_(0.25)I_(2.5)F_(1.5)

TABLE t (Cs₂SnI₄)_(1−x)(CsM′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Si, Ge,Sn, Ti, Zr, or Hf x CsI SnI₂ Cs₂SiF₆ Composition 0 2.0 1.0 0 Cs₂SnI₄ 0.11.80 0.90 0.05 Cs_(1.8)Sn_(0.9)Si_(0.05)I_(3.7)F_(0.3) 0.2 1.60 0.800.10 Cs_(1.6)Sn_(0.8)Si_(0.10)I_(3.4)F_(0.6) 0.3 1.40 0.70 0.15Cs_(1.4)Sn_(0.7)Si_(0.15)I_(3.1)F_(0.9) 0.4 1.20 0.60 0.20Cs_(1.2)Sn_(0.6)Si_(0.20)I_(2.8)F_(1.2) 0.5 1.00 0.50 0.25Cs_(1.0)Sn_(0.5)Si_(0.25)I_(2.5)F_(1.5)

TABLE u Cs(SnI₃)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ =Al, Ga, In, Sc, or Y x CsI SnI₂ AlF₃ CsF Composition 0 1.0 1.0 0 0CsSnI₃ 0.1 1.0 0.90 0.05 0.15 Cs_(1.15)Sn_(0.9)Al_(0.05)I_(2.8)F_(0.3)0.2 1.0 0.80 0.10 0.30 Cs_(1.30)Sn_(0.8)Al_(0.10)I_(2.6)F_(0.6) 0.3 1.00.70 0.15 0.45 Cs_(1.45)Sn_(0.7)Al_(0.15)I_(2.4)F_(0.9) 0.4 1.0 0.600.20 0.60 Cs_(1.60)Sn_(0.6)Al_(0.20)I_(2.2)F_(1.2) 0.5 1.0 0.50 0.250.75 Cs_(1.75)Sn_(0.5)Al_(0.25)I_(2.0)F_(1.5)

TABLE v Cs(SnI₃)_(1−x)(Cs₀M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Al xCsI SnI₂ AlF₃ Composition 0 1.0 1.0 0 CsSnI₃ 0.1 1.0 0.90 0.10CsSn_(0.9)Al_(0.10)I_(2.8)F_(0.3) 0.2 1.0 0.80 0.20CsSn_(0.8)Al_(0.20)I_(2.6)F_(0.6) 0.3 1.0 0.70 0.30CsSn_(0.7)Al_(0.30)I_(2.4)F_(0.9) 0.4 1.0 0.60 0.40CsSn_(0.6)Al_(0.40)I_(2.2)F_(1.2) 0.5 1.0 0.50 0.50CsSn_(0.5)Al_(0.50)I_(2.0)F_(1.5)

TABLE w Cs(SnI₃)_(1−x)(Cs_(0.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = P,As, Sb, V, Nb, or Ta x CsI SnI₂ CsPF₆ Composition 0 1.0 1.0 0 CsSnI₃ 0.11.0 0.90 0.05 Cs_(1.05)Sn_(0.9)P_(0.05)I_(2.8)F_(0.3) 0.2 1.0 0.80 0.10Cs_(1.10)Sn_(0.8)P_(0.10)I_(2.6)F_(0.6) 0.3 1.0 0.70 0.15Cs_(1.15)Sn_(0.7)P_(0.15)I_(2.4)F_(0.9) 0.4 1.0 0.60 0.20Cs_(1.20)Sn_(0.6)P_(0.20)I_(2.2)F_(1.2) 0.5 1.0 0.50 0.25Cs_(1.25)Sn_(0.5)P_(0.25)I_(2.0)F_(1.5)

TABLE x Cs(SnI₃)_(1−x)(CsM′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Si, Ge,Sn, Ti, Zr, or Hf x CsI SnI₂ Cs₂SiF₆ Composition 0 1.0 1.0 0 CsSnI₃ 0.11.0 0.90 0.05 Cs_(1.10)Sn_(0.9)Si_(0.05)I_(2.8)F_(0.3) 0.2 1.0 0.80 0.10Cs_(1.20)Sn_(0.8)Si_(0.10)I_(2.6)F_(0.6) 0.3 1.0 0.70 0.15Cs_(1.30)Sn_(0.7)Si_(0.15)I_(2.4)F_(0.9) 0.4 1.0 0.60 0.20Cs_(1.40)Sn_(0.6)Si_(0.20)I_(2.2)F_(1.2) 0.5 1.0 0.50 0.25Cs_(1.50)Sn_(0.5)Si_(0.25)I_(2.0)F_(1.5)

TABLE y (SnI₂)_(1−x)(Cs_(1.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Al,Ga, In, Sc, or Y x CsI SnI₂ AlF₃ CsF Composition 0 0 1.0 0 0 SnI₂ 0.1 00.90 0.05 0.15 Cs_(0.15)Sn_(0.9)Al_(0.05)I_(1.0)F_(0.3) 0.2 0 0.80 0.100.30 Cs_(0.30)Sn_(0.8)Al_(0.10)I_(1.6)F_(0.6) 0.3 0 0.70 0.15 0.45Cs_(0.45)Sn_(0.7)Al_(0.15)I_(1.4)F_(0.9) 0.4 0 0.60 0.20 0.60Cs_(0.60)Sn_(0.6)Al_(0.20)I_(1.2)F_(1.2) 0.5 0 0.50 0.25 0.75Cs_(0.75)Sn_(0.5)Al_(0.25)I_(1.0)F_(1.5)

TABLE z (SnI₂)_(1−x)(Cs_(0.5)M′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = P,As, Sb, V, Nb, or Ta x CsI SnI₂ CsPF₆ Composition 0 0 1.0 0 SnI₂ 0.1 00.90 0.05 Cs_(0.05)Sn_(0.9)P_(0.05)I_(1.8)F_(0.3) 0.2 0 0.80 0.10Cs_(0.10)Sn_(0.8)P_(0.10)I_(1.6)F_(0.6) 0.3 0 0.70 0.15Cs_(0.15)Sn_(0.7)P_(0.15)I_(1.4)F_(0.9) 0.4 0 0.60 0.20Cs_(0.20)Sn_(0.6)P_(0.20)I_(1.2)F_(1.2) 0.5 0 0.50 0.25Cs_(0.25)Sn_(0.5)P_(0.25)I_(1.0)F_(1.5)

TABLE aa (SnI₂)_(1−x)(CsM′_(0.5)F₃)_(x): 0.01 < x < 0.99, M′ = Si, Ge,Sn, Ti, Zr, or Hf x CsI SnI₂ Cs₂SiF₆ Composition 0 0 1.0 0 SnI₂ 0.1 00.90 0.05 Cs_(0.10)Sn_(0.9)Si_(0.05)I_(1.8)F_(0.3) 0.2 0 0.80 0.10Cs_(0.20)Sn_(0.8)Si_(0.10)I_(1.6)F_(0.6) 0.3 0 0.70 0.15Cs_(0.30)Sn_(0.7)Si_(0.15)I_(1.4)F_(0.9) 0.4 0 0.60 0.20Cs_(0.40)Sn_(0.6)Si_(0.20)I_(1.2)F_(1.2) 0.5 0 0.50 0.25Cs_(0.50)Sn_(0.5)Si_(0.25)I_(1.0)F_(1.5)

Examples of Type IIA materials that include monovalent organiccountercations include ab) CH₃NH₃MI₃, M=Ge, Sn, or Pb; ac) HC(NH₂)₂MI₃,M=Ge, Sn, or Pb; ad) H₃CC(NH₂)₂MI₃, M=Ge, Sn, or Pb; ae) C(NH₂)₃MI₃,M=Ge, Sn, or Pb. Examples of Type IIB materials that includeπ-conjugated divalent organic cations, such as alkyl viologen (RV; R═H,Me, Et and the like), polymeric viologens, polyaromatic pyridines, orpolyaromatic pyridine, include HVSnI₄, RVSn₂I₆ and (Acr)SnI₃. Charts 1and 2 show the structures of several organic cations that can beincluded in Type II materials.

Single crystals of the Type IIA materials can be grown directly fromsolution by leaving the precipitate inside the mother liquor for 1-2days.

For Type IIB materials, where the solid precipitates immediately afteraddition of the organic iodide, a different method is adopted: asolution of the organic iodide in aqueous HI (2 ml, 57%) is gentlylayered above a solution of MI₂ in HI (57%) and H₃PO₂ (50%) and heatedto 120° C. (6.8 ml, 4:1 volume ratio). Slow mixing of the layers affordshigh quality single-crystals over a period of 5-6 hours.

The following tables contain examples of materials and synthesisconditions for the synthesis of Type II materials:

Composition Reagent 1 Reagent 2 Colour Conditions CH₃NH₃SnI₃ SnI₂ (1.00mmol) CH₃NH₃I (1.0 mmol) Black Δ, N₂, HI, H₃PO₂ HC(NH₂)₂SnI₃ SnI₂ (1.00mmol) HC(NH₂)₂I (1.0 mmol) Black Δ, N₂, HI, H₃PO₂ CH₃C(NH₂)₂SnI₃ SnI₂(1.00 mmol) CH₃C(NH₂)₂I (1.0 mmol) Red Δ, N₂, HI, H₃PO₂ C(NH₂)₃SnI₃ SnI₂(1.00 mmol) C(NH₂)₃I (1.0 mmol) Orange Δ, N₂, HI, H₃PO₂ HVSnI₄ SnI₂(1.00 mmol) HVI₂ (0.5 mmol) Black Δ, N₂, HI, H₃PO₂ MVSnI₄ SnI₂ (1.00mmol) MVI₂ (0.5 mmol) Black Δ, N₂, HI, H₃PO₂ EVSnI₄ SnI₂ (1.00 mmol)EVI₂ (0.5 mmol) Black Δ, N₂, HI, H₃PO₂ AcrSnI₃ SnI₂ (1.00 mmol) Acr (1.0mmol) Black Δ, N₂, HI, H₃PO₂ CH₃NH₃PbI₃ PbI₂ (1.00 mmol) CH₃NH₃I (1.0mmol) Black Δ, N₂, HI, H₃PO₂ HC(NH₂)₂PbI₃ PbI₂ (1.00 mmol) HC(NH₂)₂I(1.0 mmol) Black Δ, N₂, HI, H₃PO₂ CH₃C(NH₂)₂PbI₃ PbI₂ (1.00 mmol)CH₃C(NH₂)₂I (1.0 mmol) Yellow Δ, N₂, HI, H₃PO₂ C(NH₂)₃PbI₃ PbI₂ (1.00mmol) C(NH₂)₃I (1.0 mmol) Yellow Δ, N₂, HI, H₃PO₂ CH₃NH₃GeI₃ GeI₄ (1.00mmol) CH₃NH₃I (1.0 mmol) Red Δ, N₂, HI, H₃PO₂ HC(NH₂)₂GeI₃ GeI₄ (1.00mmol) HC(NH₂)₂I (1.0 mmol) Orange Δ, N₂, HI, H₃PO₂ CH₃C(NH₂)₂GeI₃GeI₄(1.00 mmol) CH₃C(NH₂)₂I (1.0 mmol) Yellow Δ, N₂, HI, H₃PO₂C(NH₂)₃GeI₃ GeI₄ (1.00 mmol) C(NH₂)₃I (1.0 mmol) Yellow Δ, N₂, HI, H₃PO₂

Crystal data and structure refinement for CH₃NH₃GeI₃, CH₃NH₃SnI₃ andCH₃NH₃PbI₃.

Identification code CH₃NH₃GeI₃ CH₃NH₃SnI₃ CH₃NH₃PbI₃ Empirical formula CH6 I3 N Ge C H6 I3 N Sn C H6 I3 N Pb Formula weight  485.36  531.46 619.96 Temperature 293(2) K 293(2) K 293(2) K Wavelength Mo Kα 0.71073Å Mo Kα 0.71073 Å Mo Kα 0.71073 Å Crystal system Trigonal TetragonalTetragonal Space group R3m P4mm I4cm Unit cell dimensions a = 8.5534(12)Å α = a = 6.2302(9) Å α = a = 8.8494(13) Å 90° 90° α = 90° a =8.5534(12) Å β = b = 6.2302(9)) Å β = b = 8.8494(13) Å 90° 90° β = 90° c= 11.162(2) Å γ = c = 6.2316(12) Å γ = c = 12.642(3) Å 120° 90° γ = 90°Volume 707.2(2) Å³ 241.88(7) Å³ 990.0(3) Å³ Z   3   1   4 Density(calculated) 3.419 g/cm³ 3.649 g/cm³ 4.159 g/cm³ Absorption 12.983 mm⁻¹12.128 mm⁻¹ 26.312 mm⁻¹ coefficient F(000)  630  228 1040 Crystal sizeTheta range for data 3.30 to 29.05°. 3.27 to 28.94°. 2.81 to 29.03°.collection Index ranges −9 <= h <= 10, −11 <= k <= 11, −8 <= h <= 8, −8<= k <= 8, −8 <= −12 <= h <= 12, −15 <= l <= 15 l <= 8 −12 <= k <= 10,−16 <= l <= 15 Reflections collected 2203 2164 4758 Independent 506[R(int) = 0.0359] 411 [R(int) = 0.0458] 678 [R(int) = reflections0.0669] Completeness to 98.5% 98.2% 97.1% theta = 24.98° Refinementmethod Full-matrix least-squares Full-matrix least-squares on F² on F²Data/restraints/ 506/1/14 411/1/16 678/2/17 parameters Goodness-of-fiton F²   1.364   1.234   1.248 Final R indices R1 = 0.0364, wR2 = R1 =0.0325, wR2 = R1 = 0.0370, wR2 = [I > 2sigma(I)]   0.0533   0.0576  0.0820 R indices (all data) R1 = 0.079, wR2 = 0.0535 R1 = 0.0453, wR2= R1 = 0.410, wR2 = 0.0617 0.834 Absolute structure 0.03(5) 0.2(5) 0(10)parameter Extinction coefficient 0.0027(3)   0 0.00086(14) Largest diff.peak 0.637 and −0.766 e/Å⁻³ 0.864 and −0.718 e/Å⁻³ 0.967 and −1.505e/Å⁻³ and hole

Crystal data and structure refinement for HC(NH₂)₂GeI₃, HC(NH₂)₂SnI₃ andHC(NH₂)₂PbI₃.

Identification code HC(NH₂)₂GeI₃ HC(NH₂)₂SnI₃ HC(NH₂)₂PbI₃ Empiricalformula C H5 I3 N2 Ge C H5 I3 N2 Sn C H5 I3 N2 Pb Formula weight  498.36 544.46  632.96 Temperature 293(2) K 293(2) K 293(2) K Wavelength Mo Kα0.71073 Å Mo Kα 0.71073 Å Mo Kα 0.71073 Å Crystal system TrigonalTetragonal Trigonal Space group R3m P4mm P3m1 Unit cell dimensions a =8.4669(12) Å α = a = 6.3079(9) Å α = a = 8.9816(13) Å 90° 90° α = 90° a= 8.4669(12) Å β = b = 6.3079(9) Åv β = b = 8.9816(13) Å 90° 90° β = 90°c = 11.729(2) Å γ = c = 6.3087(13) Å γ = c = 11.003(2) Å 120° 90° γ =120° Volume 728.2(2) Å³ 252.02(7) Å³ 768.7(2) Å³ Z   3   1   4 Density(calculated) 3.409 g/cm³ 3.602 g/cm³ 4.102 g/cm³ Absorption 12.616 mm⁻¹11.694 mm⁻¹ 25.423 mm⁻¹ coefficient F(000)  648  234  798 Crystal sizeTheta range for data 3.28 to 29.15°. 3.23 to 29.20°. 1.85 to 29.19°.collection Index ranges −11 <= h <= 11, −10 <= k <= 11, −8 <= h <= 8, −8<= k <= 8, −12 <= h <= 12, −15 <= l <= 15 −7 <= l <= 7 −12 <= k <= 11,−15 <= l <= 15 Reflections collected 2214 2423 7475 Independent 508[R(int) = 0.0508] 391 [R(int) = 0.0210] 1645 [R(int) = reflections0.0459] Completeness to 96.3% 87.8% 99.6% theta = 24.98° Refinementmethod Full-matrix least-squares on Full-matrix least-squaresFull-matrix least- F² on F² squares on F² Data/restraints/ 508/1/17391/1/16 1645/1/30 parameters Goodness-of-fit on F²   1.131   1.217  1.112 Final R indices R1 = 0.0443, wR2 = 0.1138 R1 = 0.0261, wR2 = R1= 0.0374, wR2 = [I > 2sigma(I)] 0.0602 0.0675 R indices (all data) R1 =0.0507, wR2 = 0.1197 R1 = 0.0323, wR2 = R1 = 0.954, wR2 = 0.0627 0.879Absolute structure 0.00(13) 0.1(5) 0.25(15) parameter Extinctioncoefficient 0.0000(11) 0.026(5) 0.00026(15) Largest diff. peak 1.173 and−1.874 e/Å⁻³ 1.020 and −0.753 e/Å⁻³ 5.432 and −6.184 e/Å⁻³ and hole

Crystal data and structure refinement for C(NH₂)₃SnI₃, α-CH₃C(NH₂)₂SnI₃and β-CH₃C(NH₂)₂SnI₃.

Identification code C(NH₂)₃SnI₃ α-CH₃C(NH₂)₂SnI₃ β-CH₃C(NH₂)₂SnI₃Empirical formula C H6 I3 N3 Sn C2 H7 I3 N2 Sn C2 H7 I3 N2 Sn Formulaweight  559.48  558.49  558.49 Temperature 150(2) K 293(2) K 100(2) KWavelength Mo Kα 0.71073 Å Mo Kα 0.71073 Å Mo Kα 0.71073 Å Crystalsystem Hexagonal Hexagonal Monoclinic Space group P63/m P63mc P21 Unitcell dimensions a = 9.3124(13) Å α = a = 9.2509(13) Å α = a = 12.746(3)Å 90° 90° α = 90° a = 9.3124(13) Å β = b = 9.2509(13) Å β = b =9.2837(13) Å β = 90° 90° 112.21(3)° c = 21.230(4) Å γ = c = 15.361(3) Åγ = c = 14.709(3) Å 120° 120° γ = 90° Volume 1594.4(4) Å³ 1138.4(3) Å³1611.4(3) Å³ Z   6   4   6 Density (calculated) 3.496 g/cm³ 3.217 g/cm³3.410 g/cm³ Absorption 11.054 mm⁻¹ 10.317 mm⁻¹ 10.940 mm⁻¹ coefficientF(000)  1452  940  1936 Crystal size Theta range for data 1.92 To24.95°. 2.54 To 24.98°. 1.50 To 27.00°. collection Index Ranges −11 <= h<= 11, −11 <= k <= 11, −10 <= h <= 9, −10 <= k <= 9, −15 <= h <= 16, −25<= l <= 25 −18 <= l <= 18 −11 <= k <= 11, −18 <= l <= 18 Reflectionscollected 10355 3928 12890 Independent 971 [R(int) = 0.0990] 750 [R(int)= 0.0860] 7027 [R(int) = reflections 0.0394] Completeness to 100.0%94.3% 100% theta = 24.98° Refinement method Full-matrix least-squares onFull-matrix least-squares Full-matrix least- F² on F² squares on F²Data/restraints/ 971/0/43 750/5/30 7027/1/218 parameters Goodness-of-fiton F²   1.183   1.014   1.095 Final R indices R1 = 0.0634, wR2 = 0.1176R1 = 0.0630, wR2 = R1 = 0.0539, wR2 = [I > 2sigma(I)] 0.1081 0.1516 Rindices (all data) R1 = 0.0759, wR2 = 0.1216 R1 = 0.1342, wR2 = R1 =0.0612, wR2 = 0.1271 0.01551 Absolute structure Extinction coefficient =0.0(7) 0.32(13) parameter 0.00023(17) Largest diff. peak 1.032 and−1.371 e/Å⁻³ 0.735 and −0.572 e/Å⁻³ 4.072 and −1.683 e/Å⁻³ and hole

Crystal data and structure refinement for HVSnI₄, MVSn₂I₆, EVSn₂I₆ andAcrSnI₃.

Identification code HVSnI₄ MVSn₂I₆ EVSn₂I₆ AcrSnI₃ Empirical formula C10H10 I4 N2 Sn C12 I6 N2 Sn2 H14 C14 I6 N2 Sn2 H18 C13 H10 I3 N Sn Formulaweight  784.49  1159.2  1194.94  679.61 Temperature 293(2) K 293(2) K293(2) K 293(2) K Wavelength 0.71073 E 0.71073 Å 0.71073 E 0.71073 ECrystal system Monoclinic Trigonal Monoclinic Monoclinic Space groupC2/c P −3 P2₁/c P2₁/n Unit cell a = 17.218(3) Å α = a = 16.078 Å, α = a= 12.116(2) Å α = a = 4.6467(9) Å α = dimensions 90° 90° 90° 90° b =14.195(3) Å β = b = 16.078 Å, β = b = 13.941(3) Å β = b = 24.710(5) Å β= 116.92(3)° 90° 90.51(3)° 90.18(3)° c = 7.8515(5) Å γ = c = 25.637 Å, γ= c = 24.677(5) Å γ = c = 14.817(3) Å γ = 90° 120° 120° 120° Volume1710.9(6) Å³ 5739.338 Å³ 4167.8(14) Å³ 1701.2(6) Å³ Z   4   9   6   4Density 3.046 g/cm³ ³ 3.0174 g/cm³ 2.857 Mg/m³ 2.653 Mg/m³ (calculated)Absorption 8.695 mm⁻¹ 9.111 mm⁻¹ 8.465 mm⁻¹ 6.930 mm⁻¹ coefficientF(000) 1384  4492  3096  1216 Crystal size Theta range for 2.65 to24.99°. 1.66 to 29.23° 1.65 to 25.00°. 1.60 to 24.99°. data collectionIndex ranges −20 <= h <= 20, −18 <= h <= 20, −14 <= h <= 14, −5 <= h <=5, −15 <= k <= 16, −21 <= k <= 19, −16 <= k <= 16, −29 <= k <= 29, −9 <=l <= 8 −35 <= l <= 35 −29 <= l <= 29 −17 <= l <= 17 Reflections 514432162 26382 12633 collected Independent 1499 [R(int) = 9185 [R_(int) =7347 [R(int) = 2956[R(int) = reflections 0.0538] 0.1623] 0.1134] 0.0548]Completeness to 99.3% 98% 100.0% 98.1% theta = Refinement Full-matrixleast- Full-matrix least- Full-matrix least- Full-matrix least- methodsquares on F² squares on F² squares on F² squares on F² Data/restraints/1499/0/78 9185/0/190 7347/0/325 2956/0/164 parameters Goodness-of-fit on  1.161   1.31   0.928   1.088 F² Final R indices R1 = 0.0451, wR2 =[I > 3σ] R_(obs) = R1 = 0.0626, wR2 = R1 = 0.0483, [I > 2sigma(I)]0.0496 0.0737, wR_(obs) = 0.0733 wR2 = 0.0896 0.1063 R indices (alldata) R1 = 0.0725, wR2 = R_(all) = 0.2392, R1 = 0.1489, wR2 = R1 =0.0636, 0.0536 wR_(all) = 0.1425 0.0896 wR2 = 0.0989 Largest diff. peak0.909 and −1.024 e · Å⁻³ 12.53 and −9.23 e · Å⁻³ 0.845 and −1.153 e ·Å⁻³ 1.31 and −1.427 e · Å⁻³ and hole

Some examples of Type III materials include ah) CsMIX₂, ai) CH₃NH₃MIX₂,aj) HC(NH₂)₂MIX₂, ak) CH₃CC(NH)₂)₂MIX₂, al) C(NH)₂)₃MIX₂, where X is anycombinations of group 17 elements. The examples of specific molar ratiosof starting materials and obtained products are shown in Table ai)-ak).All numbers in table are in mmol.

TABLE ah) CsM(I_(3−x)X_(x)): M = Ge, Sn, or Pb Reagent/ Composition CsFCsCl CsBr CsI SnF₂ SnCl₂ SnBr₂ SnI₂ CsSnF₂(F_(1−x)I_(x)) x = 0.00-0.99 —— x = 0.01-1.00 x = 1.00 — — — CsSnCl₂(Cl_(1−x)I_(x)) — x = 0.00-0.99 —x = 0.01-1.00 — x = 1.00 — — CsSnBr₂(Br_(1−x)I_(x)) — — x = 0.00-0.99 x= 0.01-1.00 — — x = 1.00 — CsSnIF(F_(1−x)I_(x)) x = 0.01-0.99 — — x =0.01-0.99 x = 0.50 — — x = 0.50 CsSnICl(Cl_(1−x)I_(x)) — x = 0.01-0.99 —x = 0.01-0.99 — x = 0.50 — x = 0.50 CsSnIBr(Br_(1−x)I_(x)) — — x =0.00-0.99 x = 0.01-0.99 — — x = 0.50 x = 0.50 CsSnI₂(I_(1−x)F_(x)) x =0.01-1.00 — — x = 0.00-0.99 — — — x = 1.00 CsSnI₂(I_(1−x)Cl_(x)) — x =0.01-1.00 — x = 0.00-0.99 — — — x = 1.00 CsSnI₂(I_(1−x)Br_(x)) — — x =0.01-0.99 x = 0.00-0.99 — — — x = 1.00

TABLE ai) CH₃NH₃M(I_(3−x)X_(x)): M = Ge, Sn, or Pb Reagent/ CompositionCH₃NH₃F CH₃NH₃Cl CH₃NH₃Br CH₃NH₃I SnF₂ SnCl₂ SnBr₂ SnI₂MASnF₂(F_(1−x)I_(x)) x = 0.00-0.99 — — x = 0.01-1.00 x = 1.00 — — —MASnCl₂(Cl_(1−x)I_(x)) — x = 0.00-0.99 — x = 0.01-1.00 — x = 1.00 — —MASnBr₂(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-1.00 — — x = 1.00 —MASnIF(F_(1−x)I_(x)) x = 0.01-0.99 — — x = 0.01-0.99 x = 0.50 — — x =0.50 MASnICl(Cl_(1−x)I_(x)) — x = 0.01-0.99 — x = 0.01-0.99 — x = 0.50 —x = 0.50 MASnIBr(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-0.99 — — x =0.50 x = 0.50 MASnI₂(I_(1−x)F_(x)) x = 0.01-1.00 — — x = 0.00-0.99 — — —x = 1.00 MASnI₂(I_(1−x)Cl_(x)) — x = 0.01-1.00 — x = 0.00-0.99 — — — x =1.00 MASnI₂(I_(1−x)Br_(x)) — — x = 0.01-0.99 x = 0.00-0.99 — — — x =1.00

TABLE aj) HC(NH₂)₂M(I_(3−x)X_(x)): M = Ge, Sn, or Pb Reagent/Composition HC(NH₂)₂F HC(NH₂)₂Cl HC(NH₂)₂Br HC(NH₂)₂I SnF₂ SnCl₂ SnBr₂SnI₂ FOSnF₂(F_(1−x)I_(x)) x = 0.00-0.99 — — x = 0.01-1.00 x = 1.00 — — —FOSnCl₂(Cl_(1−x)I_(x)) — x = 0.00-0.99 — x = 0.01-1.00 — x = 1.00 — —FOSnBr₂(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-1.00 — — x = 1.00 —FOSnIF(F_(1−x)I_(x)) x = 0.01-0.99 — — x = 0.01-0.99 x = 0.50 — — x =0.50 FOSnICl(Cl_(1−x)I_(x)) — x = 0.01-0.99 — x = 0.01-0.99 — x = 0.50 —x = 0.50 FOSnIBr(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-0.99 — — x =0.50 x = 0.50 FOSnI₂(I_(1−x)F_(x)) x = 0.01-1.00 — — x = 0.00-0.99 — — —x = 1.00 FOSnI₂(I_(1−x)Cl_(x)) — x = 0.01-1.00 — x = 0.00-0.99 — — — x =1.00 FOSnI₂(I_(1−x)Br_(x)) — — x = 0.01-0.99 x = 0.00-0.99 — — — x =1.00

TABLE ak) CH₃C(NH₂)₂ M(I_(3−x)X_(x)): M = Ge, Sn, or Pb Reagent/Composition CH₃C(NH₂)₂F CH₃C(NH₂)₂Cl CH₃C(NH₂)₂Br CH₃C(NH₂)₂I SnF₂ SnCl₂SnBr₂ SnI₂ MFSnF₂(F_(1−x)I_(x)) x = 0.00-0.99 — — x = 0.01-1.00 x = 1.00— — — MFSnCl₂(Cl_(1−x)I_(x)) — x = 0.00-0.99 — x = 0.01-1.00 — x = 1.00— — MFSnBr₂(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-1.00 — — x = 1.00— MFSnIF(F_(1−x)I_(x)) x = 0.01-0.99 — — x = 0.01-0.99 x = 0.50 — — x =0.50 MFSnICl(Cl_(1−x)I_(x)) — x = 0.01-0.99 — x = 0.01-0.99 — x = 0.50 —x = 0.50 MFSnIBr(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-0.99 — — x =0.50 x = 0.50 MFSnI₂(I_(1−x)F_(x)) x = 0.01-1.00 — — x = 0.00-0.99 — — —x = 1.00 MFSnI₂(I_(1−x)Cl_(x)) — x = 0.01-1.00 — x = 0.00-0.99 — — — x =1.00 MFSnI₂(I_(1−x)Br_(x)) — — x = 0.01-0.99 x = 0.00-0.99 — — — x =1.00

TABLE al) C(NH₂)₃M(I_(3−x)X_(x)): M = Ge, Sn, or Pb Reagent/ CompositionC(NH₂)₃F C(NH₂)₃Cl C(NH₂)₃Br C(NH₂)₃I SnF₂ SnCl₂ SnBr₂ SnI₂GUSnF₂(F_(1−x)I_(x)) x = 0.00-0.99 — — x = 0.01-1.00 x = 1.00 — — —GUSnCl₂(Cl_(1−x)I_(x)) — x = 0.00-0.99 — x = 0.01-1.00 — x = 1.00 — —GUSnBr₂(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-1.00 — — x = 1.00 —GUSnIF(F_(1−x)I_(x)) x = 0.01-0.99 — — x = 0.01-0.99 x = 0.50 — — x =0.50 GUSnICl(Cl_(1−x)I_(x)) — x = 0.01-0.99 — x = 0.01-0.99 — x = 0.50 —x = 0.50 GUSnIBr(Br_(1−x)I_(x)) — — x = 0.00-0.99 x = 0.01-0.99 — — x =0.50 x = 0.50 GUSnI₂(I_(1−x)F_(x)) x = 0.01-1.00 — — x = 0.00-0.99 — — —x = 1.00 GUSnI₂(I_(1−x)Cl_(x)) — x = 0.01-1.00 — x = 0.00-0.99 — — — x =1.00 GUSnI₂(I_(1−x)Br_(x)) — — x = 0.01-0.99 x = 0.00-0.99 — — — x =1.00

The materials were prepared by mixing appropriate amounts of SnX₂, CsX,CH₃NH₃X, HC(NH₂)₂X, H₃CC(NH₂)₂X or C(NH₂)₃X in several ratios, as listedbelow (X can be F, Cl, Br or I). The solids were placed in borosilicatetubes and placed in a sand bath preheated to 250° C. The obtained meltswere cooled in air. Reaction time varied between 1-5 minutes, increasingas a function of the molecular weight of X. When longer reaction timeswere required to obtain the melt (>2 min), the solids were heated upunder a gentle flow of N₂.

Examples of Type V materials are any possible combination of Type I-IVmaterials. For example, (AMX₃)_(1−x)(M′O_(z))_(x) and(AMX₃)_(1−x)(AM′F₆)_(x). An extreme example of this type of materials isthe melt of “CsSnF₂I” composition, which attacks its reaction container(SiO₂ or Al₂O₃) to form mixed-metal materials. The PL intensity of thematerials thus observed can be optimized by varying the annealing time.Incorporating two metal shifted wavelengths of emission spectra as shownin AM_(1−x)M′_(x)X₃, (AMX₃)_(1−x)(A′M′X′₃)_(x) and(AM_(1-z)X_(3-z))(M′X_(z)). The transition metal halides can be used toalter the wavelengths of emission spectra.

Properties of A/M/X Metal Halide Compounds that Emit Photoluminescencein the Visible and Near Infrared Regions.

I) Photo-Response

A sample was placed in a sample holder, exposed to the laser and its PLintensity was measured. This was the starting point. Without turning offthe beam, PL intensity was recorded as a function of irradiation time atspecified intervals. After the last measurement during irradiation thelaser was shut down and the sample was left in the dark. A final PLintensity measurement was made after about 1 hour.

The photoluminescence spectra of various embodiments of the A/M/Xcompounds are shown in FIGS. 1, 3, 5(A)-(C), 7(B), 8, 10, 12(A)-(B),14(A)-(B), 16, 18, 20, and 24 (A)-(B). Corresponding absorbance spectraof various embodiments of the A/M/X compounds are shown in FIGS. 2, 4,6, 7(A), 9, 11, 13, 15, 17, 19, and 25 (A)-(D).

It was consistently observed that the materials responded to prolongedexposure to light. Depending on the particular materials the initiallyobserved PL intensity was either gaining or losing strength duringirradiation until it reached its maximum effect at which point PLintensity stabilized. When the laser was shut off, the material slowlyrelaxed back to its original state prior to light excitation. Somematerials were classified based on the effect of light on the PLintensity. The classification is the following:

i) Negative photoresponse (the PL intensity decreases as a function ofirradiation): Materials that typically display such an effect areASnI_(x)Cl_(y) and particularly so as the y/x ratio increases, in whichcase the effect is more pronounced.

ii) Positive photoresponse (the PL intensity decreases as a function ofirradiation): Materials that typically display such an effect are(CsSnI₃)_(1−x)(SnO)_(x) compositions (particularly for x>0.5) and APbX₃materials.

iii) Irregular photoresponse (the PL intensity changes as a function ofirradiation but not in a specific manner): Materials that typicallydisplay such an effect are generally Cs(SnI₃)_(1−x)(CsSi_(0.5)F₃)_(x)compositions and, in general, most of the compounds containing F.

II) Photo-Reactivity

A sample was placed in a sample holder, exposed to the laser and its PLintensity was measured. This was the starting point. PL intensity wasrecorded as a function of irradiation time. After irradiation, thesample was removed and examined for color changes on the spot ofirradiation.

Some of the materials described above, particularly Type III materialsprepared by synthetic process 3, were prepared as a yellow modificationwhich has a weak PL intensity or is totally PL-inactive. Irradiationcaused a color change in the material which was accompanied with anenhancement of PL emission. The transformation was reversible and thematerial relaxed back to its originally prepared state. The effect wasmore pronounced for mixed I/F compositions.

Solution-Based Thin Film Preparation of A/M/X Compounds and Type I, II,III, IV and V Systems Exhibiting Photoluminescence.

Classes of compounds A/M/X (A=alkali metals, organic cations; M=Group14; X=Group 17) and Type I, II, III, IV and V systems exhibitingphotoluminescence were found to be soluble in polar organic solventssuch as N,N-dimethylformamide, acetonitrile and their mixtures to givetransparent solutions. The color of solutions was dependent on that ofthe solids: CsSnI₃ gave yellow solutions and CsSnCl₃ yielded colorlesssolutions. Based on this observation, the solution-based thin filmpreparation method of materials described in Part 1 was developed.

Some characteristics of the compounds and systems are: (1) the systemsare highly soluble in polar organic solvents to give viscous,transparent solutions; (2) by employing such a dissolution properties, asolution-processed thin film preparation method of those materials byspin-coating or spraying was developed; (3) such method was generallyapplicable to all relevant compounds of the A/M/X systems; (4) Becausethis was a low-temperature process at 120-150° C., various cheapsubstrates of plastics can be used, which are flexible and favorable formany applications.

Many members of A/M/X system undergo phase-change driven by temperature.CsSnI₃ is a complex example that has two different phases at RT: yellowone-dimensional and black perovskite phases. The black phase has twomore high temperature phases by a distortion of perovskite framework.Only the black phase emits desirable photoluminescence (PL) at 950 nm atRT. Accordingly, stabilization of black phase in a thin film form wasdesired.

Recently, synthesis and characterization of polycrystallineCsSnI_(3−x)Cl_(x) thin films have been reported using a vacuumdeposition technique. They were prepared by a two-step method. First,multiple layers of SnI₂ (or SnCl₂) and CsI were alternately deposited invacuum (˜10⁵ torr) on glass, Si and ceramic substrates by a combinationof thermal and e-beam evaporators. The resulting films were the yellowphase and did not emit PL at 950 nm. Then, thermal annealing wasrequired to activate PL emission over 350° C. This method has severaldrawbacks. Firstly, it requires expensive high vacuum procedures.Secondly, the evaporation method does not provide precise chemicalcomposition control. Thus doping to optimize desirable performance ischallenging. Thirdly, the deposited films need to be annealed at hightemperature at 350° C. for PL activity.

The present solution-processing procedure is advantageous in that thinfilms of the materials immediately form the black phase afterspin-coating and exhibit strong PL emission at 950 nm at RT. Similarly,in the case of CsSnCl₃, the present spin coating process stabilizes thehigh temperature phase of the yellow perovskite structure rather thanthe room temperature phase of white one-dimensional structure. Thesolution-processed deposition procedure is highly reproducible,applicable to large-area substrates, available in designed doping foroptimizing properties and incomparably cheaper than vacuum techniques.By finishing preparation at low temperature, cheap and flexible plasticsubstrates are usable. The typical thin films of CsSnI₃ emits 21-foldstronger PL at 950 nm than the commercial available InP:S singlecrystalline wafer.

EXAMPLES Example 1 Preparation CsSnI₃ Thin Films by a Solution Process

High purity CsSnI₃ ingots were obtained by reacting a stoichiometricmixture of CsI and SnI₂ in an evacuated fused silica tube at 500-550° C.for 1 h, followed by a cooling to RT for 3 h. Powders of CsSnI₃(˜120-350 mg) were dissolved in polar organic solvents (1 mL), such asanhydrous N,N-dimethylformamide (DMF) and acetonitrile or their mixturesto give yellow transparent solution. The solutions were filtered througha 0.2 μm syringe filter, and diluted to various concentrations. Forbetter films, Sn/O/Cl buffer layers were pre-deposited on substrates.Highly crystalline thin films were deposited by a spin-coating processat 1000 rpm for 40 s, followed by drying at 125° C. for 5 min on a hotplate under N₂ atmosphere. The resulting films were black in color andcould be deposited on various substrates such as Si, glass and flexibleplastics.

The X-ray powder diffraction pattern of the film matched well with thatof a theoretical calculation of black orthorhombic phase of CsSnI₃.Scanning electron microscopy (SEM) images of CsSnI₃ thin films showedcontinuous surface morphology that consists of several hundred nm toseveral micron size grains (FIG. 21). Electron dispersive spectroscopy(EDS) on films gave a close chemical composition to the expected value.The cross-sectional image of CsSnI₃ films on Si substrate bytransmission electron microscopy showed that the thin films arepolycrystalline.

The thin films of Cs/Sn/I/Cl phases were deposited on the glasssubstrates coated with a reflecting layer that consists of alternatingSiO₂ and TiO₂ layers. The CsSnI₃ films exhibited 42-fold stronger PLintensity at ˜950 nm to single crystalline InP:S wafer. The CsSnI₂Cl andCsSn₂I₃Cl₂ films exhibited 15 and 7-fold stronger PL intensity to InP:Swafer. The solution-processed film emitting strong PL also can beprepared by a spray method. The thin films of Cs₂SnI₂Cl₂ could beobtained by spraying the as a solution dissolved in DMF. The resultingfilms exhibited strong PL at 950 nm.

The same procedure can be applied to other members of the families ofmaterials described here. Broad applications for materials exhibitingphotoluminescence at RT include light emitting diodes, sensors,anti-counterfeiting, inventory, photovoltaics, solar cells and otherapplications.

EXPERIMENTAL

Materials Synthesis

1. Solid state reaction. Pure target materials were prepared by reactinga stoichiometric mixture of alkali metal halide (organic halide) andmetal halide starting materials under vacuum in a borosilicate or fusedsilica tube at 450° C. for 1 h. For organic metal halide compounds, astoichiometric mixture was reacted at 200° C. for 2 h. The sameprocedure was applied when metal oxides were used. Reacting mixtureswere sometimes pre-reacted by grinding with mortar and pestle.

2. Solid state reaction in an open container: Stoichiometric mixtures ofstarting materials were loaded in a pyrex test tube (VWR, about 15 mmID). The materials were shaken mechanically to ensure a ratherhomogenous mixture of the materials. The tube was placed on a sandbathheated at specified temperature, typically 450° C. under N₂, reacteduntil a homogeneous melt formed (typically 1-5 min) and quenched to air.The reaction was repeated several times to obtain homogeneous products.It is very important to keep the reaction time very short, becausebesides atmospheric oxygen coming in, there may also be gas evolutionwhich is volatile SnX₄.

3. Solution preparation: A two-necked flask (usually 50 or 100 mL) ischarged with the corresponding Sn(II), Pb(II) or Ge(IV) halide or oxide,and a mixture of conc. HI 57%(6.8 ml, 7.58M) and conc. H₃PO₂ 50% (1.7ml, 9.14M). Note: the volumes come from the fact that a Pasteur pipettecontains 1.7 ml of solution, so it is 4:1 ratio in “Pasteur pipette”units. The mixture is flushed with N₂ gas for 1-2 minutes (through thesolution) to remove most of the oxygen and a constant flow is maintainedthroughout the process (outside of the solution, for crystal growthpurposes). For Ge, add double the volume (or sometimes more) of H₃PO₂ toensure full reduction. For Pb, H₃PO₂ may be omitted. Typically, thescale is 1 mmol based on the Ge, Sn (or Pb) halide. For Ge, it isrequired to add some EtOH (˜10 ml) to dissolve GeX₄ halides. Then, underconstant magnetic stirring, the mixture is heated at ˜130 C by immersingthe flask in a silicon oil bath, until a clear, bright yellow solutionis obtained. If it is slightly orange then add more H₃PO₂ to make ityellow. The temperature can change (up or down) depending on specificconditions (e.g. isolation of both the kinetic and the thermodynamicproduct). Once the yellow solution is obtained, add the alkalimetal/organic iodide. This can be added either as solid or as a solutiondepending on its solubility. Typically, AI₂ salts form insolubleproducts, whereas AI salts form soluble products (there are of coursesome exceptions!). If the addition produces a solid, I collect it byfiltration (fritted glass, the solvent attacks paper!!). Otherwise, aclear yellow solution should be obtained. The solvent is evaporated byheating (b.p. is about 120 C) under N₂ flow. At a certain point ofconcentration (about 4 ml, but this varies, depending on the cation) asolid separates. The solution is cooled to room temperature and thesolid is filtered (fritted glass, the solvent attacks paper!!). Theyields range from 50-100%. If high yield is required concentratefurther. The solid is washed with an organic solvent (EtOH is almostalways a good washing solvent) and dried under vacuum in air. The solidis usually stable when dry for 1-2 days. However, when in contact withthe mother liquor exposed in air, the product decomposes (oxidation,hydrolysis, iodolysis, etc. . . . ) within minutes! A clear indicationof oxidation is a deeply red (from [I₃]⁻) or black ([SnI₆]²⁻) color ofthe solution. In this case, more H₃PO₂ should be added. The wholeprocess can last from 10 min to 3 h. High quality materials wereprepared by dissolving equimolar amounts of GeI₄, SnI₂ or PbI₂ (1 mmol)and CH₃NH₃I, HC(NH₂)₂I, H₃CC(NH₂)₂I or C(NH₂)₃I (1 mmol), respectively,in a solvent mixture comprising of concentrated aqueous solutions of HI(57%) and H₃PO₂ (50%) to a total of 6.8 ml (4:1 volume ratio). Thesolution was heated up to boiling and slowly concentrated toapproximately half its volume, where a fine solid precipitates. Thewhole process is performed under a blanket of N₂. Yields vary between50-100%.

Crystal Growth. Typically, the precipitated solid contains X-ray qualitysingle crystals. If not, there are two possibilities, depending on thesolubility of the product. Before that, a general piece of advice is toreduce the quantities of the solids that you add. If this does not workthen:

a) if a clear solution is obtained, then switch of the stirring, reducethe N₂ flow so that it does not disturb the solution (alternatively, addmore H₃PO₂, remove the N₂ flow and cap the flask tightly) andconcentrate slowly (˜100 C is good practice). When a solid separates,increase heating (to about 150-170 C) and add dropwise HI until thesolid redissolves. If in the process the solution oxidizes add some moreH₃PO₂ to reduce it. Once a clear solution is obtained, turn of the heatand leave the solution for crystallization overnight.

b) if a solid precipitates immediately, then dissolve AI or AI₂ into asuitable solvent (avoid hydrolysable (MeCN) or basic (DMF) solvents andalso don't use amines with (CH₃)₂CO!). Do not add this solution directlyto the yellow solution. First of all, turn of the stirring and reducethe N₂ flow as much as possible. Then with a pipette or syringe, add alayer of a solvent on top of the aqueous layer so that mixing occursslowly (within 5 minutes) and turn off the heat. The crystals thatprecipitate/grow on cooling are significantly better/larger than theones obtained with the standard method.

Physical Measurements.

1. X-Ray Powder Diffraction. Analyses for powder samples were performedusing a calibrated CPS 120 INEL X-ray powder diffractometer (Cu Kαradiation) operating at 40 kV/20 mA and equipped with aposition-sensitive detector with flat sample geometry.

2. X-ray Thin Film Diffraction. Grazing incident angle X-ray diffraction(GIAXRD) scans were measured with a Rigaku ATX-G Thin-Film DiffractionWorkstation using Cu Kα radiation coupled to a multilayer mirror. Thescan rate and interval were set to 2°/min and 0.10° for 2θ scan.

3. Electron Microscopy. Semiquantitative analyses of the compounds wereperformed with a JEOL JSM-35C scanning electron microscope (SEM)equipped with a Tracor Northern energy dispersive spectroscopy (EDS)detector. SEM images were taken with a Hitachi S-3400N-IIvariable-pressure SEM. TEM investigations were carried out in JEOL 2100Fmicroscope operating at a 200 keV accelerating voltage.

4. Solid-State UV-Vis spectroscopy. Optical diffuse reflectancemeasurements were performed at room temperature using a Shimadzu UV-3101PC double-beam, double-monochromator spectrophotometer operating in the200-2500 nm region. The instrument is equipped with an integratingsphere and controlled by a personal computer. BaSO₄ was used as a 100%reflectance standard. The sample was prepared by grinding the crystalsto a powder and spreading it on a compacted surface of the powderedstandard material, preloaded into a sample holder. The reflectanceversus wavelength data generated, were used to estimate the band gap ofthe material by converting reflectance to absorption data according toKubelka-Munk equations: α/S=(1−R)²/(2R), where R is the reflectance andα and S are the absorption and scattering coefficients, respectively.

5. Raman Spectroscopy. Raman spectra were recorded on a Holoprobe Ramanspectrograph equipped with a CCD camera detector using 633 nm radiationfrom a HeNe laser for excitation and a resolution of 4 cm⁻¹. Laser powerat the sample was estimated to be about 5 mW, and the focused laser beamdiameter was ca. 10 μm. A total of 128 scans was sufficient to obtaingood quality spectra.

6. Infrared Spectroscopy. FT-IR spectra were recorded as solids in a CsIor KBr matrix. The samples were ground with dry CsI or KBr into a finepowder and pressed into translucent pellets. The spectra were recordedin the far-IR region (600-100 cm⁻¹, 2 cm⁻¹ resolution) and mid-IR region(500-4,000 cm⁻¹, 2 cm⁻¹ resolution) with the use of a Nicolet 6,700FT-IR spectrometer equipped with a TGS/PE detector and silicon beamsplitter.

7. Differential Thermal Analysis (DTA). Experiments were performed onShimadzu DTA-50 thermal analyzer. A sample (˜30 mg) of groundcrystalline material was sealed in a silica ampoule under vacuum. Asimilar ampoule of equal mass filled with Al₂O₃ was sealed and placed onthe reference side of the detector. The sample was heated to 850° C. at10° C. min⁻¹, and after 1 min it was cooled at a rate of −10° C. min⁻¹to 50° C. The residues of the DTA experiments were examined by X-raypowder diffraction. Reproducibility of the results was confirmed byrunning multiple heating/cooling cycles. The melting and crystallizationpoints were measured at onset of endothermic and exothermic peaks.

3.9. X-Ray Crystallography.

Intensity data were collected in the various range of temperature from100(2)-500(2) K on a STOE IPDS II and 2T diffractometer with Mo Kαradiation operating at 50 kV and 40 mA with a 34 cm diameter imagingplate. Individual frames were collected with a 4 min exposure time and a1.0 co rotation. The X-AREA, X-RED, and X-SHAPE software package wasused for data extraction and integration and to apply empirical andanalytical absorption corrections. The SHELXTL software package was usedto solve and refine the structure.

Example 2 Synthesis and Characterization of Organic-Inorganic HybridMetal Halides

Experimental.

Synthesis.

Sn granules, I₂ (99.8%), HC(NH₂)₂Cl (98%), Pb(NO₃)₂ (99.999%), KI(99.0%) and CsI (99.95%) solids and distilled HI 57% (99.95%), H₃PO₂ 50%and CH₃NH₂ 40% aqueous solutions and MeONa 25% methanolic solution werepurchased from Sigma-Aldrich.

SnI₂ was prepared, as follows. A 3-necked 500 mL round bottom flask wascharged with 50 g granulated Sn and 300 mL of 2M aqueous HCl was added.The mixture was flushed with N₂ for 1-2 min. 70 g of I₂ were added intwo portions. The solution turned to dark brown. The flask was immersedin a silicon oil bath heated to 170° C. and attached to a condenser. Atthis point the N₂ flow should be at a minimum, in order to avoid I₂fumes to escape or crystallize inside the condenser. When the atmosphereinside the flask was free of I₂ vapors, 50 mL of 2M HCl was added, totransfer the quantity of I₂ that has crystallized inside the condenserback into the flask. At this point, the N₂ flow was increased slightly.As the reaction proceeded the color of the solutions changedprogressively to red and finally to yellow. When the color of thesolution turned yellow (took 60-120 minutes), Sn was added to themixture at 15 min time intervals in ˜1 g batches. This was because itwas necessary to remove any traces of I₂ or, less likely, O₂ present.The reaction was judged to be complete when the color of the solutionwas bright yellow and the Sn added retained its luster for 15 minutes.The yellow solution was transferred by decantation (since Sn isgranulated filtering was not required) to a 500 mL 2-necked flask, whichwas immersed in a water bath heated at 70° C. under a flow of N₂. Thehot yellow solution was slowly brought to ambient temperature and thefirst crystals of SnI₂ crystallized as long red needles. This coolingstep encouraged the formation of larger crystals which facilitate theisolation of the material. The precipitation was complete on standingovernight. After 1 day, large red needles of SnI₂ were formed. Theproduct was isolated either by filtration, followed by copious washingwith 0.01M degassed HCl and drying in a vacuum oven at 50° C., orthrough evaporation to dryness at 70° C. The yield was 90-100 g (90-99%based on total I₂) of pure SnI₂. The dried solid had a bright red colorand was stable on standing in dry air for several weeks. Combined actionof oxygen and water on the material resulted in rapid oxidation toorange SnI₄, while distilled water caused hydrolysis to white Sn(OH)₂.

PbI₂ was prepared by mixing aqueous solutions of Pb(NO₃)₂ and KI in 1:2molar ratio. CH₃NH₃I was prepared by neutralizing equimolar amounts ofHI and aqueous CH₃NH₂. HC(NH₂)₂I was prepared by neutralizing EtOHsolutions of HC(NH)(NH₂) (prepared by neutralizing HC(NH₂)₂Cl with MeONain MeOH and discarding NaCl) with aqueous HI. The purity of thematerials was confirmed by powder diffraction measurements.

A) Solution Synthesis.

General procedure: A 100 ml 2-necked round bottom flask was charged witha mixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml,9.14M). The liquid was degassed by passing a stream of nitrogen throughit for 1 min and keeping it under a nitrogen atmosphere throughout theexperiment. SnI₂ (372 mg, 1 mmol) or PbI₂ (462 mg, 1 mmol) was dissolvedin the mixture upon heating the flask to 120° C. using an oil bath,under constant magnetic stirring, forming a bright yellow solution.

CH₃NH₃SnI₃ (1): A 100 ml 2-necked round bottom flask was charged with amixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml, 9.14M).The liquid was degassed by passing a stream of nitrogen through it for 1min and keeping it under a nitrogen atmosphere throughout theexperiment. SnI₂ (372 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid CH₃NH₃I (159 mg, 1 mmol) which dissolved immediately.The solution was evaporated to approximately half its original volume byheating at 120° C. The stirring was discontinued and the solution wasleft to cool back to room temperature. Upon cooling, black, elongated,rhombic dodecahedral (12 faces) crystals of the title compound wereprecipitated. The crystals were left to grow inside the mother liquorfor a further 24 h under a nitrogen atmosphere before being filtered andwashed copiously with degassed EtOH. Yield 70-90%. (Diffuse-ReflectanceInfrared Fourier Transformed spectroscopy, DRIFT) KBr: 3393br, 3191br,2961w, 2925w, 2855w, 1623s, 1471m, 1383w, 1320w, 1260m, 1124s, 1061s,806w, 566w. On exposure to the atmosphere, the lustrous black crystalsdeveloped some iridescence on their surface in the first few minutes ofexposure before gradually converting into black/greenish dull solidafter approximately 24 h.

HC(NH₂)₂SnI₃ (2): A 100 ml 2-necked round bottom flask was charged witha mixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml,9.14M). The liquid was degassed by passing a stream of nitrogen throughit for 1 min and keeping it under a nitrogen atmosphere throughout theexperiment. SnI₂ (372 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid HC(NH₂)₂I (172 mg, 1 mmol), which dissolved immediately.The solution was evaporated to approximately half its original volume byheating at 120° C. The stirring was discontinued and the solution wasleft to cool back to room temperature. Upon cooling, black rhombicdodecahedral crystals (12 faces) of the title compound wereprecipitated. The crystals were left to grow inside the mother liquorfor a further 24 h under a nitrogen atmosphere before being filtered andwashed copiously with degassed EtOH. Yield 70-90%. DRIFT, KBr: 3402br,3269br, 2926w, 2925w, 1714s, 1624w, 1400m, 1111m, 1065s, 810, 598w. Onexposure to the atmosphere, the lustrous black crystals developed someiridescence on their surface in the first few minutes of exposure beforegradually converting into black/greenish dull solid after approximately24 h.

CH₃NH₃PbI₃ (3): A 100 ml 2-necked round bottom flask was charged with amixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml, 9.14M).The liquid was degassed by passing a stream of nitrogen through it for 1min and keeping it under a nitrogen atmosphere throughout theexperiment. PbI₂ (462 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid CH₃NH₃I (159 mg, 1 mmol), which dissolved immediately.The solution was evaporated to approximately half its original volume byheating at 120° C. The stirring was discontinued and the solution wasleft to cool back to room temperature. Upon cooling, black, rhombicdodecahedral crystals (12 faces) of the title compound precipitated. Thecrystals were left to grow inside the mother liquor for a further 24 hunder a nitrogen atmosphere before being filtered and washed copiouslywith anhydrous EtOH. Yield 70-90%. DRIFT, KBr: 3529br, 3180br, 2956w,2925w, 2711w, 1599s, 1471m, 1250m, 1056m, 960m, 910m, 492m. The crystalswere insensitive on exposure to the atmosphere but spontaneouslyhydrolyzed to yellow PbI₂ upon wetting in H₂O. They retained theirluster for several weeks, after which time their surface becomes dull,without, however, affecting the integrity of the solid.

HC(NH₂)₂PbI₃ (4a): A 100 ml 2-necked round bottom flask was charged witha mixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml,9.14M). The liquid was degassed by passing a stream of nitrogen throughit for 1 min and keeping it under a nitrogen atmosphere throughout theexperiment. PbI₂ (462 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot solution wasadded solid HC(NH₂)₂I (172 mg, 1 mmol), which dissolved immediately. Thesolution was evaporated to approximately half its original volume byheating at 120° C. The stirring was discontinued and the solution wasleft at 100° C. to evaporate slowly. Black hexagonal (8 faces) ortrigonal (5 faces) crystals of the title compound were precipitated andgrown at this temperature. After standing for 2-3 h at 100° C., under anitrogen atmosphere, the temperature was set to 80° C. for a further 2-3h. The step was repeated two more times to reach 60° C. and 40° C. atwhich point the solution was left to come to room temperature bypowering off the hotplate. The crystals were collected by filtration andwashed copiously with anhydrous EtOH. Yield 50-60%. DRIFT, KBr: 3345br,2938s, 2858s, 1691s, 1656s, 1543m, 1139s, 1066s, 608m, 430w. Uponexposure to the atmosphere, at ambient temperature, the compound totallyconverts to 4b within 1 h. Hydrolyses in water to PbI₂.

HC(NH₂)₂PbI₃ (4b): A 100 ml 2-necked round bottom flask was charged witha mixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml,9.14M). The liquid was degassed by passing a stream of nitrogen throughit for 1 min and keeping it under a nitrogen atmosphere throughout theexperiment. PbI₂ (462 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid HC(NH₂)₂I (172 mg, 1 mmol), which dissolved immediately.The solution was evaporated to approximately half its original volume byheating at 120° C. The stirring was discontinued and the solution wasleft at 100° C. to evaporate slowly. Black hexagonal (8 faces) ortrigonal (5 faces) crystals were precipitated. At that point, heatingwas discontinued and the mixture was cooled to room temperature. After4-5 h the black crystals fully converted to yellow ones of essentiallythe same shape. The yellow crystals were collected by filtration andwashed copiously with anhydrous EtOH. Yield 50-60%. The crystals wereinsensitive on exposure to the atmosphere but spontaneously hydrolyzedto yellow PbI₂ upon wetting in H₂O.

CH₃NH₃Sn_(1−x)Pb_(x)I₃ solid solutions (5): A 100 ml 2-necked roundbottom flask was charged with a mixture of aqueous HI (6.8 ml, 7.58M)and aqueous H₃PO₂ (1.7 ml, 9.14M). Mixtures of SnI₂ (279 mg, 0.75 mmol,5a; 186 mg, 0.50 mmol, 5b; and 93 mg, 0.25 mmol, 5c), and PbI₂ (116 mg,0.25 mmol, 5a; 231 mg, 0.50 mmol, 5b; and 347 mg, 0.75 mmol, 5c) weredissolved, forming a bright yellow solution which was heated to 120° C.using an oil bath. To the hot solution was added solid CH₃NH₃I (159 mg,1 mmol), which dissolved immediately. The solution was evaporated toapproximately half its original volume by heating at 120° C. Thestirring was discontinued and the solution was left to cool back to roomtemperature. Upon cooling, black, rhombic dodecahedral crystals (12faces) or truncated octahedral (14 faces), depending on thecrystallization temperature or/and the composition, precipitated. Thecrystals were left to grow inside the mother liquor for a further 24 hunder a nitrogen atmosphere before being filtered and washed copiouslywith degassed EtOH. The solid solutions reflected the properties oftheir parent compounds. Sn-containing solids were air sensitive andPb-containing ones were readily hydrolyzable.

CsSnI₃ (6a): A 100 ml 2-necked round bottom flask was charged with amixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml, 9.14M).The liquid was degassed by passing a stream of nitrogen through it for 1min and keeping it under a nitrogen atmosphere throughout theexperiment. PbI₂ (462 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid CsI (260 mg, 1 mmol), which dissolved immediately. 2 minafter the addition of the yellow needle-shaped crystals started toprecipitate. The stirring was discontinued and the solution was left tocool to room temperature. The crystals were left to grow inside themother liquor for a further 24 h under a nitrogen atmosphere beforebeing filtered and washed copiously with degassed EtOH. With this samplethere was almost always a Cs₂SnI₆ (9b) impurity which formedspontaneously upon addition of solid CsI into the solution. Pure yellowneedles can be obtained by adding a solution of CsI in 1 mL ofconcentrated HI over the course of 2 min. Yield 80-90%. The compound wasair sensitive as demonstrated by the full conversion of the yellowcrystals into a black solid within 24 h. The black solid is 6b.

Cs₂SnI₆ (6b): A 100 ml 2-necked round bottom flask was charged with amixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml, 9.14M).The liquid was degassed by passing a stream of nitrogen through it for 1min and keeping it under a nitrogen atmosphere throughout theexperiment. SnI₂ (372 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid CsI (260 mg, 1 mmol), which dissolved immediately. 2 minafter the addition of the yellow needle-shaped crystals started toprecipitate. The precipitate was dissolved by adding 10 mL of acetone,producing a dark red solution. After 5 min the stirring was discontinuedand the solution was left to cool to room temperature. Black truncatedoctahedral (14 faces) crystals were formed upon standing. The crystalswere left to grow inside the mother liquor for a further 24 h under anitrogen atmosphere before being filtered and washed with degassed EtOH.Yield 80-90%. The compound is air- and water-stable. It formed blackaqueous solutions which decomposed only slowly to deposit white SnO₂ anddeep red I₃ ⁻ solutions.

CsPbI₃ (7): A 100 ml 2-necked round bottom flask was charged with amixture of aqueous HI (6.8 ml, 7.58M) and aqueous H₃PO₂ (1.7 ml, 9.14M).The liquid was degassed by passing a stream of nitrogen through it for 1min and keeping it under a nitrogen atmosphere throughout theexperiment. PbI₂ (462 mg, 1 mmol) was dissolved in the mixture uponheating the flask to 120° C. using an oil bath, under constant magneticstirring, forming a bright yellow solution. To the hot yellow solutionwas added solid CsI (260 mg, 1 mmol), which dissolved immediately,producing a yellow precipitate. The precipitate was redissolved byadding the minimum required amount of acetone ˜5 mL. Stirring wasdiscontinued and the solution was left to cool to room temperature.Yellow needle-shaped crystals were formed upon standing. The crystalswere left to grow inside the mother liquor for a further 24 h under anitrogen atmosphere before being filtered and washed copiously withanhydrous EtOH. Yield 80-90%. The compound was insensitive to air but ithydrolyzed spontaneously.

B) Solid State Reactions

1) Sealed tube preparations: Equimolar amounts of MI₂ and AI were loadedin a 9 mm pyrex tube. Typically, the scale is 1 mmol based on the Sn orPb halide. The materials were shaken mechanically to ensure a homogenousmixture. The tube was placed on a sealing line, evacuated to 10⁴ mbar,and flame sealed. The tube was immersed on a sand bath standing a 200°C., in such a way that the mixture of solids was heated homogeneously.About 4/5 of the tube was maintained outside the bath at roomtemperature. The solids were left in the bath for 2 h forming ahomogeneous black solid. The Sn-containing solids were air sensitivewhereas the Pb-containing ones were air stable. No obvious color changeswere observed.

2) Open tube preparations: Equimolar amounts of MI₂ and AI were loadedin a 15 mm pyrex test tubes. The materials were shaken mechanically toensure a homogenous mixture. The tube was immersed in a sand bathstanding at 350° C. under a gentle flow of nitrogen. The reactionproceeded within 0.5-1 min. The formation of a homogeneous black meltsignaled the end of the reaction. Upon melting the tube was removed fromthe bath and left to cool in air, usually producing a shiny black ingot.The melt was formed for the Sn-containing compounds only. Pb-containingsolids decomposed on prolonged heating (>3 min) or by raising thetemperature above 400° C., through evolution of I₂ gas, crystallizing onthe cooler walls of the tube. The shiny black ingots of theSn-containing compounds were gradually converted into dullblack/greenish solids within 24 h.

3) Room temperature solid state preparations: Equimolar amounts of MI₂and AI were placed in an agate mortar and ground carefully with a pestleuntil a visually homogeneous, black powder was obtained. TheSn-containing solids were air sensitive whereas the Pb-containing oneswere air stable. No obvious color changes were observed.

Characterization.

Thermal Analysis: The Thermogravimetric Analysis (TGA) measurements wereperformed on a Shimadzu TGA-50 thermogravimetric analyzer in aluminumboats under a N₂ flow. Differential Thermal Analysis (DTA) was performedon a Shimadzu DTA-50 thermal analyzer in aluminum boats using α-Al₂O₃ inidentical aluminum boats as a reference, under a N₂ flow. DifferentialScanning calorimetry (DSC) measurements were performed on a DSC-60calorimeter in aluminum boats using identical aluminum boats as areference. The temperature range studied was 20-500° C.

Scanning Electron Microscopy (SEM): Semiquantitative microprobe analysesand energy-dispersive spectroscopy (EDS) were performed with a HitachiS-3400 scanning electron microscope equipped with a PGTenergy-dispersive X-ray analyzer. Data were acquired with anaccelerating voltage of 20 kV.

Optical Spectroscopy: Optical diffuse-reflectance measurements wereperformed at room temperature using a Shimadzu UV-3101 PC double-beam,double-monochromator spectrophotometer operating from 200 to 2500 nm.BaSO₄ was used as a non-absorbing reflectance reference. A Nicolet 6700IR spectrometer equipped with a diffuse-reflectance kit was used for the4000-400 cm⁻¹ spectral region. The spectrum was referenced towards ametallic mirror used as the non-absorbing reflectance standard. Thegenerated reflectance-versus-wavelength data were used to estimate theband gap of the material by converting reflectance to absorbance dataaccording to the Kubelka-Munk equation: α/S=(1−R)²/2R, where R is thereflectance and R and S are the absorption and scattering coefficients,respectively.

Photoluminescence Measurements: The samples were measured using anOmniPV Photoluminescence system, equipped with a DPSS frequency-doublingNd:YAG laser (500 mW power output, class 4) emitting at 532 nm coupledwith a bundle of 8 400 μm-core optical fibers as an excitation source.The emission was measured in the 200-1700 nm range with two StellarNetspectrometers, covering the 200-850 nm (CCD) and 900-1700 nm (InGaAs)areas, coupled with 400 μm-core optical fibers and integrated within thebundle (10 optical fibers total). The backscattered light was collectedat ˜180° angle at 3 mm from the sample. For estimating the intensity ofthe signal and also to account for the excitation beam fluctuations, adisk of single-crystalline InP (930 nm emission peak) was measuredbefore and after the sample measurements. The excitation time used was100 ms and the data were averaged for 5 independent scans. The data wereanalyzed using the SpectraWiz or the OmniPV PL software.

Vibrational Spectroscopy: Fourier-transformed infrared spectra (FT-IR)were recorded on a Nicolet 6700 IR spectrometer in the 400-4000 cm⁻¹spectral region using a diffuse-reflectance kit. The measurement wasconducted under a constant flow of nitrogen. The spectrum is referencedtowards a metal mirror used as the 100% reflectance standard.

Charge Transport Properties:

a) Low Temperature Resistivity Measurements

Four-probe low temperature measurements were performed on a QuantumDesign PPMS device in 5-330K temperature range using the four-probetechnique on well-defined rhombic dodecahedral single-crystals grownfrom the solution method. Current was applied on a rhombic face of thecrystal and the voltage was measured on two of the apices of thepolyhedron (c-axis). The resistance was recorded as an average of bothnegative and positive current polarities.

b) High Temperature Resistivity and Seebeck Coefficient and Hall-effectMeasurements

Four-probe room and high temperature measurements were performed in the300-550K range. Measurements were made for arbitrary current directionsin the ab-plane using standard point contact geometry. A home-maderesistivity apparatus equipped with a Keithley 2182A nanovoltometer,Keithley 617 electrometer, Keithley 6220 Precision direct current (DC)source, and a high temperature vacuum chamber controlled by a K-20 MMRsystem was used. Data acquisition was controlled by custom writtensoftware.

Seebeck measurements were performed in the 300-400K temperature range onthe same home-made apparatus using Cr/Cr:Ni thermocouples as electricleads that were attached to the sample surface by means of colloidalgraphite isopropanol suspension. The temperature gradient along thecrystal was generated by a resistor on the ‘hot’ side of the crystal.The data were corrected for the thermocouple contribution using a copperwire. Data acquisition was controlled by custom written software.

Powder X-ray diffraction studies: Powder X-ray diffraction measurementswere performed using a silicon-calibrated CPS 120 INEL powder X-raydiffractometer (Cu Kα, 1.54056A°) graphite monochromatized radiation)operating at 40 kV and 20 mA, equipped with a position-sensitivedetector with a flat sample geometry.

Single crystal X-ray diffraction studies: Single-crystal X-raydiffraction experiments were performed using either a STOE IPDS II orIPDS 2T diffractometer using Mo Kα radiation)(λ=0.71073A°) and operatingat 50 kV and 40 mA. Integration and numerical absorption correctionswere performed using the X-AREA, X-RED, and X-SHAPE programs. Allstructures were solved by direct methods and refined by full-matrixleast squares on F² using the SHELXTL program package. The TwinRotMat,Rotax and other PLATON functions of the WinGX platform were used toidentify the twinning domains and the correct space group, respectively,for the low temperature crystal structures. The JANA2006 software wasemployed for the refinement of the multiple twinning domains present atlow temperature structures. The ISOTROPY software was used to calculatethe group-subgroup relationships of the perovskite structures and tofind the correct space group.

Results and Discussion.

Synthetic aspects: The synthesis of the materials can be accomplishedusing a variety of approaches. This includes “crude” methods such asdirect mortar and pestle grinding of starting materials at roomtemperature in air, high temperature fusion of the precursors in silicatubes and solution growth of single crystals. All methods produce thetitle compounds, though the purity and properties of the resultingmaterials vary significantly with the method of synthesis. The CsMI₃analogues were also prepared and used as reference materials forcomparison with the AMI₃ systems.

The materials obtained from the grinding method usually were a mixtureof products, the main phase (visually homogeneous black solid) being thedesired perovskite, with significant amounts of unreacted precursors asevidenced by the X-ray powder patterns of the solids. This method wasfast and efficient and is the method of choice to observe the unusualoptical properties of each compound. The solids obtained through thismethod consistently produced the most intense room temperaturephotoluminescence signal, presumably due to the large degree of defectspresent or due to interface effects (vide infra).

Another method that gives the perovskite phases quickly was by heatingthe starting materials together at ˜350° C. in air or under a gentleflow of nitrogen. A pure black melt was obtained in all cases after0.5-1 min and on solidification produced a shiny black material. It isimperative to keep the reaction time short to prevent the volatilizationCH₃NH₃I and SnI₄. The latter can be seen on the cooler parts of the tubeas an orange liquid. HC(NH₂)₂I also decomposes at that temperature tothe volatile sym-triazine and NH₄I. The materials prepared in thisfashion were obtained in higher yield compared to the ones prepared atroom temperature by grinding but the presence of oxidation products(i.e., SnI₄) and the loss of some of the organic cation rendered thesematerials defective, not following the exact stoichiometry. The presenceof oxidized impurities was evident by thermogravimetric analysis (to bediscussed in detail later) by following the mass loss anomalies in the150-200° C., where SnI₄ was expected to melt and volatilize. Forcomparison, the Pb compounds did not show any mass loss in this region(PbI₄ was not known). Instead, the oxidation mechanism on prolongedheating for the Pb solids (in air) produced purple vapor of I₂ withsubsequent formation of PbO in the solid.

Using solid state techniques we were able to isolate the materials inpure form by grinding the precursors together, followed by subsequentannealing at 200° C. inside vacuum-sealed pyrex tubes for ˜2 h. Again,some condensation was seen on the cooler parts of the tube (mainlyformation of the white organic iodide salt and/or orange SnI₄ crystalsin Sn-containing compounds at the sand bath-air interface) therefore thematerials prepared by this method were expected to be stoichiometricallydefective as well, but contain no oxidation byproducts compared to thosefrom the open tube methods.

The method of choice to obtain pure materials with more exactstoichiometries was the solution method. Through this route we were ableto obtain high quality single crystals that allowed X-ray crystalstructure characterization. The crystals of the obtained materials werewell defined a could be modified depending on the temperature profile ofthe precipitation. Namely, the products vary from polycrystallineaggregates when precipitation started at elevated temperatures (ca. 130°C.) under vigorous stirring to discrete polyhedral crystals whencrystallization occurred by cooling slowly to room temperature. Thecrystals had a strong tendency towards a rhombic dodecahedral geometry,in general, though each phase adopted a unique habit; thus, 1 and 3adopted the elongated rhombic dodecahedron (rhombo-hexagonaldodecahedron) shape, 2 adopted a more regular rhombic dodecahedron,while 4a adopted the hexagonal prism shape.

The materials obtained from the aqueous HI/H₃PO₂ solution mixture wereexceptionally pure as judged by XRD. This was mainly attributed to thereducing nature of phosphinic acid which was sufficient to suppress anyoxidation occurring by reducing I₃ ⁻ (formed by aerial oxidation of thesolution) back to I⁻. This was evident from the disappearance of thedark red color of the triiodide ion from solution (when oxidationoccurred) which turned bright yellow when treated with phosphinic acid.The purging effects of phosphinic acid ensured that no stray oxidationproducts were present during precipitation, while no traceablebyproducts were found as a result of the presence of H₃PO₂/H₃PO₃ speciesin the reaction mixture. In most cases the sole product of the reactionwas a pure black crystalline phase. Interestingly, in the case ofHC(NH₂)₂PbI₃ we were able to isolate two isomers; a black perovskitephase (4a) and a yellow hexagonal phase (4b) having a different crystalstructure. It appears that the black phase 4a is stable at hightemperatures (>60° C.) while at lower temperatures the black phase fullyconverts to yellow phases inside the mother liquor. When the productsare separated from the solvent and dried they interconvert only slowly.

A complication that occurred when trying to isolate pure 4a, was thepartial decomposition of formamidine to ammonia and sym-triazine in thestrongly acidic reaction media using a prolonged reaction time. Whereasthe sym-triazine did not interfere in the reaction, the ammonium cationslightly complicates the reaction. NH₄PbI₃.2H₂O, occurring as paleyellow needles, or, as yellow needles, could be detected in 4 as animpurity. The effect can be minimized by keeping the reaction time asshort as possible and by avoiding temperatures >150° C.

It is important to point out that in the case of the Cs analogs CsSnI₃and CsPbI₃ the solution method cannot be employed for the preparation ofthe black forms of these phases and the yellow phases are isolatedinstead. The yellow modifications of CsSnI₃ (6a) and CsPbI₃ (7), areisostructural and crystallized in long, thin, needle-shaped crystals, inaccordance with the 1-D structure of these materials. The solutionmethod was also capable of producing the black Cs₂Sn^(IV)I₆ phase (6b)an oxidized byproduct in the synthesis of CsSnI₃ with a molecular saltstructure.

The stability of the materials in air varied. The Pb-containing phaseswere stable in air for months, though they were affected by humidity andlost their crystalline luster after a couple of weeks. However, this wasonly a surface effect since the bulk properties of the materials wereretained. The Sn-containing materials were air and moisture sensitiveand partially decomposed within 2 h before total decomposition after 1d; this behavior was detected in the powder diffraction patternevolution over time, from the patterns showing the appearance of extraBragg diffraction peaks that belonged to the oxidized Sn^(IV) species,(CH₃NH₃)₂SnI₆, as judged by comparing the diffraction pattern of theunindexed peaks with those of Cs₂SnI₆. A general trend observed was thatmethylammonium-containing materials were considerably more stable thanformamidinium-containing ones. This reflects the respective stability ofthe cations themselves and particularly the tendency of HC(NH₂)₂ ⁺ todissociate to ammonia and sym-triazine.

Structure Description.

Single crystals of the materials obtained from the solution method wereanalyzed by single crystal X-ray diffraction studies. The diffractionmeasurements were performed at room temperature, and in the range100-400 K to determine the structural changes that occur during theseveral phase transitions. The presence of phase transitions isconsistent with the perovskite nature of the materials. The common trendfor 1-4 was that there were two phase transitions that took place,leading to three distinct structural modifications. These wereclassified as, the high temperature α-phase, the intermediatetemperature β-phase and the low temperature γ-phase, while thenon-perovskite yellow phase is termed the δ-phase for consistency withthe nomenclature used in the studies of CsSnI₃. The δ-phase denotes an1D array of octahedra and it occurs in δ-HC(NH₂)₂PbI₃(4b).

α-phase structures: The α-phase for 1 and 2 was found at roomtemperature and consisted of pseudo-cubic unit cell with latticeparameter being a=6.2307| and a=6.3290 Å, respectively. The α-phase for3 was also pseudo-cubic and formed after a phase transition thatoccurred at temperature >50° C. with lattice parameter a=6.3130 Å.Compound 4a was unique in adopting a trigonal phase with a=8.9817 Å andc=11.0060 Å at room temperature. We are as yet uncertain if this is theα-phase for 4a or if there is yet another higher symmetry phase athigher temperatures. For simplicity, here we refer to 4a as the α-phase.

Because phase transitions affect the photoluminescence properties ofCsSnI₃ (which adopts the orthorhombic γ-phase at room temperature), wedecided to investigate the phase transitions that occur in the presenthybrid perovskites at different temperatures. The main differencebetween CsMI₃ inorganic perovskites and the organic hybrid perovskitesis the possibility of obtaining the ideal cubic Pm-3m structure. Whereasin the case of Cs⁺ a single atom occupies the 1b Wyckoff position in thecenter of the cube of the perovskite structure in a primitive unit cell,in the cases of CH₃NH₃ ⁺ and HC(NH₂)₂ ⁺ this is not possible in thisspace group. As a consequence, these cations tend to be disordered inthe cage. In a cubic unit cell this requires one atom to be placedexactly on the special position and the other(s) to be disordered aroundthe 12 possible orientations inside the cube. Such a situation is notparticularly appealing on chemical grounds, especially if one considersthe hydrogen bonding capabilities of both CH₃NH₃ ⁺ and HC(NH₂)₂ ⁺.Moreover, when a polar organic cation is confined inside the cage itsdipole moment cannot be cancelled out, thus inducing non-centrosymmetricdomains in the crystal.

With this in mind, in our consideration of the crystal structuredeterminations when an ambiguity occurs we prefer thenon-centrosymmetric space group as the correct choice. Of course, whensupercells arise from the phase transition an inversion center mightemerge which will cancel the dipole moments. At room temperature, thecrystal structures of 1-4 are non-centrosymmetric. The space groups thatwe have found, except for β-CH₃NH₃PbI₃ which already has a supercell atthis temperature, are classified into the ‘ferroelectric distortion’category, which means that they are subgroups of the ideal Pm-3m cubicstructure by abolishing certain symmetry elements including the centerof symmetry. In this case the symmetry elements are lost as a result ofthe displacement of the B cation from the center of the octahedron,though the ferroelectric displacement is only marginal. In P4 mm, whichis adopted by the higher temperature phase of CH₃NH₃SnI₃ and CH₃NH₃PbI₃the displacement occurs along the c-axis (the tetragonal axis), while inAmm2, the space group adopted by HC(NH₂)₂SnI₃ at high temperature, the Matom displacement occurs along the a and b axes.

α-HC(NH₂)₂PbI₃ is a special case because its space group(centrosymmetric P-3 m1) seems to be the parent phase for the family ofperovskites termed ‘1:2 B cation ordering’. The peculiarity arises fromthe fact that there is only one type of B-cation —Pb. Following thisdistortion, there are three crystallographically inequivalent Pb cationsinside the unit cell.

CH₃NH₃PbI₃ adopts the β-phase at room temperature which is distorted(see below) and for this reason we sought to solve the structure of theα-phase using the same specimen by raising the temperature above thereported transition at ˜330K. We solved the crystal structure ofα-CH₃NH₃PbI₃ by measuring a crystal at 400K and found it to beisostructural to α-CH₃NH₃SnI₃. For the collection of good datasets wefound it essential for the crystal to be maintained at the collectiontemperature for at least 2 h (to complete the phase transition) beforestarting the data collection.

β-phase structures: Attempts to determine the crystal structures of thehybrid materials, revealed multiple twinning on lowering ofcrystallographic symmetry. This happens when moving from high to lowsymmetry distorting the unit cell. In order for the crystal to complywith such a distortion it “splits” into several domains, a fact thatcertainly complicates the structure determination. All typicalpseudomerohedral twin domains characteristic of the perovskite structurewere observed during structure refinement, along the [010], [101]and[121] lattice directions. In all cases the effect was fully reversibleand when the temperature was raised the crystal was completelyreassembled. Another interesting point here is that when we move fromthe low to the higher symmetry phase during a phase transition, thiseffect is not so severe, as it was shown, for example, in the case ofCH₃NH₃PbI₃. Effectively, this behavior suggests that the transition issmoother on lowering the symmetry and more abrupt when increasing it, atrend that is confirmed by the transport measurements.

The temperature range where the α-phase converts to the β-phase liesbetween 300-150K for the hybrid perovskites, with the exception ofCH₃NH₃PbI₃ where the transition takes place at 333K. Lowering of thesymmetry is achieved by means of tilting of the octahedra. Pureoctahedral tilting starting from an ideal cubic Pm-3m structure followsa specific sequence of descending symmetry leading to a certain set ofpossible space groups. Such a situation is described using the Glazernotation. This notation describes the distortion about a, b, c axesusing the axis symbol accompanied by a ‘+’ or ‘-’ symbol to denote ifthe tilting between two adjacent polyhedral is in-phase or out-of-phase,respectively. Following the space group sequence for CsSnI₃, startingwith the ideal Pm-3m structure, it follows a phase transition at ˜425Kto the tetragonal structure, P4/mbm (double cell volume), beforeobtaining an orthorhombic structure, Pnma (quadruple volume) below 351K.Such a sequence can be described by considering a pure perovskitetilting model, ignoring ferroelectric-type displacements, with anin-phase distortion along c-axis initially and distortions along the a-and b-axes in the orthorhombic phase. The situation for the hybridperovskites appears to be different because both ferroelectric andtilting distortions should be taken into consideration. Crystallographicdata for the α- and β-phases of 1-4 are given in the three followingtables summarizes the crystallographic data for the related compounds4b, 5b, 6b and 7.

Crystallographic Data of the α-Phases for 1-4

Compound α- α-CH₃NH₃SnI₃ α-HC(NH₂)₂SnI₃ CH₃NH₃PbI₃ α-HC(NH₂)₂PbI₃ (1)(2) (3) (4a) Temperature 293(2) K 340(2) K 400(2) K 293(2) K Crystalsystem Tetragonal Orthorhombic Tetragonal Trigonal Space group P4mm Amm2P4mm P3m1 Unit cell a = 6.2302(10) a = 6.3286(10) Å, a = 6.3115(2) a =8.9817(13)Å, dimensions Å, α = 90.00° Å, α = 90.00° α = 90.00° b =8.9554(11) Å, α = 90.00° b = 8.9817(13) Å, b = 6.2302(10) β = 90.00° b =6.3115(2) β = 90.00° Å, c = 8.9463(11) Å, Å, c = 11.006(2) Å, β = 90.00°γ = 90.00° β = 90.00° γ = 120.00° c = 6.2316(11) c = 6.3161(2) Å, Å, γ =90.00° γ = 90.00° Volume 241.88(7) Å³ 507.03(12) Å³ 251.60(2) Å³768.9(2) Å³ Z   1   2   1   3 Density 3.649 g/cm³ 3.566 g/cm³ 4.092g/cm³ 4.101 g/cm³ (calculated) Absorption 12.128 mm⁻¹ 11.579 mm⁻¹ 25.884mm⁻¹ 25.417 mm⁻¹ coefficient F(000)  228  468  260  798 Crystal size(mm⁻³) 0.053 × 0.032 × 0.176 × 0.163 × 0.035 × 0.022 × 0.041 × 0.026 ×  0.021   0.136   0.017   0.021 θ range 3.27 to 28.94° 3.22 to 29.11°3.23 to 24.92° 1.85 to 29.19° Index ranges −8 <= h <= 8, −8 <= h <= 8,−7 <= h <= 7, −12 <= h <= 12, −8 <= k <= 8, −10 <= k <= 12, −6 <= k <=6, −12 <= k <= 11, −8 <= l <= 8 −12 <= l <= 10 −7 <= l <= 7 −15 <= l <=15 Reflections 2164 2490 1628 7475 collected Independent 411 [R_(int) =686 [R_(int) = 0.0517] 300 [R_(int) = 1645 [R_(int) = reflections0.0458] 0.0700] 0.0459] Completeness to θ θ = 28.94°, θ = 29.11°, 100% θ= 24.92°, θ = 29.19°, 99.6% 98.2% 98.2% Data/restr./ 411/2/16 686/3/24300/2/19 1645/7/44 param. Goodness-of-fit   1.262   0.861   1.176  0.908 Final R indices R_(obs) = 0.0327, R_(obs) = 0.0323, R_(obs) =0.0417, R_(obs) = 0.0398, [>2σ(I)] wR_(obs) = 0.0576 wR_(obs) = 0.0577wR_(obs) = wR_(obs) = 0.0666 0.0981 R indices [all R_(all) = 0.0451,R_(all) = 0.0615, R_(all) = 0.0417, R_(all) = 0.0892, data] wR_(all) =0.0613 wR_(all) = 0.0632 wR_(all) = 0.0981 wR_(all) = 0.0807 2^(nd) twindomain [−1 0 0 0 0 1 0 1 [0 −½ −½ −1 −½ ½ −1 [−1 0 0 0 0 1 0 [−1 0 0 0−1 0 0 0 0] ½ −½] 43(3)% 1 0] 36(3)% 1] 49(69)% Flack parameter [−1 0 00 −1 0 0 0 [−1 0 0 0 −1 0 0 0 −1] [−1 0 0 0 −1 0 0 6.36(2)% −1] −50(32)%0 −1] 12(3)% [−1 0 0 0 −1 0 0 0 −1] −4(45)% 20(15)% 2^(nd) domain Flack[1 0 0 0 0 1 0 1 [0 −½ −½ −1 ½ ½ −1 [1 0 0 0 0 1 0 1 not refinedparameter 0] ½ ½] 0] 3(68)% 13(3)% 37(4)% Extinction — 0.0200(4)0.016(2) — coefficient Largest diff. 0.895/−0.861 e · Å⁻³ 0.394/−0.952 e· Å⁻³ 1.871/−2.008 e · Å⁻³ 1.104/−1.727 e · Å⁻³ peak/hole Wavelength0.71073 Å Refinement Full-matrix least-squares on F² method R =Σ||F_(o)| − |F_(c)||/Σ|F_(o)|, wR = {Σ[w(|F_(o)|² −|F_(c)|²)²]/Σ[w(|F_(o)|⁴)]}^(1/2) and calc w = 1/[σ²(Fo²) + (0.0262P)² +0.0000P] where P = (Fo² + 2Fc²)/3

Crystallographic Data of the β-Phases for 1-4

Compound β- β-CH₃NH₃SnI₃ β-HC(NH₂)₂SnI₃ CH₃NH₃PbI₃ β-HC(NH₂)₂PbI₃ (1)(2) (3) (4a) Temperature 200(2) K 180(2) K 293(2) K 150(2) K Crystalsystem Tetragonal Orthorhombic Tetragonal Trigonal Space group I4cm Imm2I4cm P3 Unit cell a = 8.7577(15) a = 12.5121(9) Å, a = 8.849(2) Å, a =17.7914(8) Å, dimensions Å, α = 90.00° α = 90.00° α = 90.00° α = 90.00°b = 12.5171(8) Å, b = 8.849(2) Å, b = 17.7914(8) Å, b = 8.7577(15) β =90.00° β = 90.00° β = 90.00° Å, c = 12.5099(9) Å, c = 12.642(2) c =10.9016(6) Å, β = 90.00° γ = 90.00° Å, γ = 120.00° c = 12.429(3) Å, γ =90.00° γ = 90.00° Volume 953.2(3) Å³ 1959.2(2) Å³ 990.0(4) Å³ 2988.4(3)Å³ Z   4   8   4   12 Density 3.703 g/cm³ 3.692 g/cm³ 4.159 g/cm³ 4.221g/cm³ (calculated) Absorption 12.309 mm⁻¹ 11.986 mm⁻¹ 26.312 mm⁻¹ 26.159mm⁻¹ coefficient F(000)  912 1872 1040  3192 Crystal size (mm⁻³) 0.158 ×0.111 × 0.018 × 0.014 × 0.005 × 0.003 × 0.038 × 0.030 ×   0.089   0.009  0.002   0.023 θ range 4.65 to 24.94° 2.30 to 29.15° 3.26 to 29.03°1.87 to 25.00° Index ranges −9 <= h <= 10, −17 <= h <= 17, −12 <= h <=12, −21 <= h <= 20, −10 <= k <= 9, −17 <= k <= 17, −12 <= k <= 10, −21<= k <= 21, −14 <= l <= 14 −17 <= l <= 17 −16 <= l <= 15 −12 <= l <= 12Reflections 3058 9462 4483 19512 collected Independent 521 [R_(int) =2876 [R_(int) = 678 [R_(int) = 7026 [R_(int) = reflections 0.1007]0.0464] 0.0664] 0.0596] Completeness to θ θ = 24.94°, θ = 29.15°, 99.7%θ = 29.03°, θ = 25.00°, 100% 99.2% 97.1% Data/restr./ 521/2/21 2876/1/47678/2/18 7026/18/192 param. Goodness-of-fit   1.142   0.963   1.268  0.839 Final R indices R_(obs) = 0.0637, R_(obs) = 0.0486, R_(obs) =0.0378, R_(obs) = 0.0656, [>2σ(I)] wR_(obs) = 0.1442 wR_(obs) = 0.1276wR_(obs) = wR_(obs) = 0.1584 0.0865 R indices [all R_(all) = 0.0639,R_(all) = 0.0780, R_(all) = 0.0418, R_(all) = 0.1780, data] wR_(all) =0.1443 wR_(all) = 0.1432 wR_(all) = 0.0879 wR_(all) = 0.2133 2^(nd) twindomain [−½ ½ ½ ½ −½ ½ [0 0 −1 0 −1 0 −1 0 [−1 0 0 0 1 0 0 [−1 0 0 0 −1 00 0 1 1 0] 0] 0 −1] 50(4)% 1] 94.19(2)% 7.9(2)% 19.74(6)% Flackparameter [−1 0 0 0 −1 0 0 0 [−1 0 0 0 −1 0 0 0 −1] [−1 0 0 0 −1 0 0 [−10 0 0 −1 0 0 0 −1] −1] 0 −1] −34(23)% 9(5)% 0(10)% 51(5)% 2^(nd) domainFlack [½ ½ ½ ½ ½ ½ 1 [0 0 −1 0 1 0 −1 0 not refined not refinedparameter 1 0] 0] 16.7(2)% 23.78(6)% Extinction 0.00247(6) 0.000808(18)— 0.000498(8) coefficient Largest diff. 2.002/−1.194 e · Å⁻³2.606/−1.191 e · Å⁻³ 0.927/−1.676 e · Å⁻³ 3.391/−4.496 e · Å⁻³ peak/holeWavelength 0.71073 Å Refinement Full-matrix least-squares on F² method R= Σ||F_(o)| − |F_(c)||/Σ|F_(o)|, wR = {Σ[w(|F_(o)|² −|F_(c)|²)²]/Σ[w(|F_(o)|⁴)]}^(1/2) and calc w = 1/[σ²(Fo²) + (0.0262P)² +0.0000P] where P = (Fo² + 2Fc²)/3

Crystallographic Data of the β-Phases for 4b, 5b, 6b and 7

Compound δ- β- HC(NH₂)₂PbI₃ CH₃NH₃Sn_(0.46)Pb_(0.54)I₃ α-Cs₂SnI₆ (4b)(5b) (6b) δ-CsPbI₃ (7) Temperature 293(2) K 293(2) K 293(2) K 293(2) KCrystal system Hexagonal Tetragonal Cubic Orthorhombic Space group P6₃mcI4cm Fm-3m Pnma Unit cell a = 8.6603(14) a = 8.8552(6) Å, a = 11.6276(9)a = 10.4342(7) dimensions Å, α = 90.00° Å, Å, α = 90.00° α = 90.00° b =8.8552(6) Å, α = 90.00° b = 4.7905(3) Å, b = 8.6603(14) β = 90.00° b =11.6276(9) β = 90.00° Å, c = 12.5353(12) Å, Å, c = 17.7610(10) β =90.00° γ = 90.00° β = 90.00° Å, γ = 90.00° c = 7.9022(6) Å, c =11.6276(9), γ = 120.00° Å, γ = 90.00° Volume 513.27(12) Å³ 982.95(13) Å³1572.1(2) Å³ 887.78(10) Å³ Z   2   4   4   4 Density 4.096 g/cm³ 3.870g/cm³ 4.842 g/cm³ 5.393 g/cm³ (calculated) Absorption 25.384 mm⁻¹ 19.219mm⁻¹ 17.925 mm⁻¹ 33.373 mm⁻¹ coefficient F(000)  532  964 1912 1184Crystal size 0.062 × 0.046 × 0.191 × 0.160 × 0.109 0.071 × 0.053 × 0.265× 0.022 × (mm⁻³)   0.031   0.044   0.013 θ range 2.72 to 24.99° 2.82 to29.01° 3.03 to 28.96° 2.26 to 29.16° Index ranges −10 <= h <= 7, −10 <=h <= 12, −15 <= h <= 15, −14 <= h <= 14, −9 <= k <= 10, −11 <= k <= 11,−14 <= k <= 15, −5 <= k <= 6, −8 <= l <= 9 −17 <= l <= 17 −15 <= l <= 15−22 <= l <= 24 Reflections 1687 4728 3871 7985 collected Independent 354[R_(int) = 788 [R_(int) = 0.0738] 143 [R_(int) = 1330 [R_(int) =reflections 0.0759] 0.0595] 0.0733] Completeness to θ θ = 24.99°, θ =29.01°, 100% θ = 28.96°, θ = 29.16°, 99% 100% 100% Data/restr./ 354/3/17788/2/22 143/0/7 1330/0/32 param. Goodness-of-fit   1.055   1.162  1.483   1.069 Final R indices R_(obs) = 0.0437, R_(obs) = 0.0678,R_(obs) = 0.0317, R_(obs) = 0.0266, [<2σ(I)] wR_(obs) = 0.1066 wR_(obs)= 0.1732 wR_(obs) = wR_(obs) = 0.0457 0.0697 R indices [all R_(all) =0.0516, R_(all) = 0.0809, R_(all) = 0.0322, R_(all) = 0.0364, data]wR_(all) = 0.1102 wR_(all) = 0.1809 wR_(all) = 0.0698 wR_(all) = 0.04772^(nd) twin domain [−1 −1 0 0 1 0 0 0 [−½ ½ −½ ½ −½ −½ −1] — — −1] −1 0]25(1)% 39(7)% Flack parameter [−1 0 0 0 −1 0 0 0 [−1 0 0 0 −1 0 0 0 −1]— — −1] 28(5)% 0(3)% 2^(nd) domain not refined [½ ½ −½ ½ ½ −½ −1 −1 — —Flack parameter 0] 22(1)% Extinction 0.0081(15) 0.00084(10) 0.00247(12)0.0127(3) coefficient Largest diff. 1.975/−0.721 e · Å⁻³ 2.980/−2.175 e· Å⁻³ 1.005/−1.241 e · Å⁻³ 0.887/−1.986 e · Å⁻³ peak/hole Wavelength0.71073 Å Refinement Full-matrix least-squares on F² method R =Σ||F_(o)| − |F_(c)||/Σ|F_(o)|, wR = {Σ[w(|F_(o)|² −|F_(c)|²)²]/Σ[w(|F_(o)|⁴)]}^(1/2) and calc w = 1/[σ²(Fo²) + (0.0262P)² +0.0000P] where P = (Fo² + 2Fc²)/3

Both CH₃NH₃SnI₃ and CH₃NH₃PbI₃ distort in a similar manner in theirβ-phase, showing out-of-phase tilting of the polyhedra combined withferroelectric type of off-centering along the c-axis adopting thetetragonal I4 cm space group. The ferroelectric displacement componentis once again microscopic effecting a slight polarization of theoctahedra along the c-axis. The tilting angle is 17.4° and 16.4° forCH₃NH₃SnI₃ and CH₃NH₃PbI₃ as measured at 150K and 293K, respectively. At180K the framework of HC(NH₂)₂SnI₃ is found to be orthorhombic Imm2. Theonset of the transition is at approximately 250K. It displays two majordistortions at 9.7° and 11.2° and a minor one at 2.2°. HC(NH₂)₂PbI₃ at150K adopts the P3 trigonal space group following a quadrupling of theroom temperature cell volume. There is a broad array of distortions, dueto significant off-centering of Pb, roughly averaged between the valuesof 9.0° and 11.5°. For good measure, the 373K distortion of β-CsSnI₃ is18.

It was apparent that the methylammonium compounds have a tendency todistort the octahedra in the out-of-phase mode, whereas formamidiniumphases seem to distort in-phase, the latter being similar to β-CsSnI₃.Contrary to the purely inorganic phase, however, the distortions lead tolower symmetry of the lattice whence the tilting of the octahedra isless severe.

γ-phase structures: As the temperature continues to decrease furthersymmetry lowering occurs leading to the γ-phases. The γ-phase isencountered close to 100K, but for γ-CH₃NH₃PbI₃ this temperature wasfound to be at ˜150K. The γ-phases were easily recognizable fromdiffraction studies because of an intense broadening and splitting ofthe Bragg peaks in addition to the extra spots that appear. This can beclearly seen in the precession images generated from diffraction datacollected at 100K, as compared to those created from room temperaturedata. We were unable to solve the structure of the γ-phases mostlybecause about 50% of the observed peaks remain unindexed during the unitcell determination. The unindexed peaks result from the formation ofmultiple twin domains which are created by the “freezing” of the cations(and subsequently the perovskite lattice) in different orientations.From our attempts to solve the crystal structures, we estimate that theγ-phases adopt a monoclinic crystal system. The methylammonium phases ofSn and Pb seem to acquire a C-centered cell whereas the formamidiniumperovskites seem to adopt a primitive cell. It is however necessary tohighlight a distinction between the methylammonium and formamidiniumbehavior; whereas the γ-phase of the former contains many split peaks,for the latter the splitting is much less intense and insteadsuperlattice ordering was observed leading to multiplication of theparent unit cell. This may be attributed to the lower point groupsymmetry of CH₃NH₃ ⁺(C_(3v)) as opposed to that of HC(NH₂)₂ ⁺(C_(2v)),with the latter being more “flexible” to comply with the latticedistortions.

δ-phase structures: Interestingly, the δ-phase of HC(NH₂)₂PbI₃ ismarkedly different from the δ-phase occurring in CsSnI₃ and CsPbI₃; inthe latter case the structure comprises a double chain of MI₆ ⁴⁻octahedra. The intra-chain linking of the polyhedra in the individualchains is affected by edge-sharing of square bipyramidal units. Theoctahedral coordination occurs by interchain bonding by a relativelylonger M-I bond with an iodide occupying a basal position of the squarepyramid, overall adopting the NH₄CdCl₃-structure type. δ-HC(NH₂)₂PbI₃ onthe other hand, consists of face-sharing octahedra propagating along the001 direction forming single chains, with the cations occupying thespace between the chains, thus adopting a nearly hcp-lattice (P6₃mc).The deviation from the ideal symmetry can be attributed to the HC(NH₂)₂⁺ cations which lack a center of inversion despite the fact that thenitrogen atoms where treated as disordered during the structuredetermination process. At this point we should stress that theblack-to-yellow transformation is a reversible effect. For example, whena single crystal of δ-HC(NH₂)₂PbI₃ was heated at 400K for ˜2 h apolycrystalline black solid was formed instead, though not in asingle-crystal-to single-crystal manner. Interestingly, while the α- toδ-phase transformation can be repeated indefinitely in the mother liquorby running heating and cooling cycles in the absence of a liquid mediumthe transformation was irreversible; i.e. the polycrystalline blackmaterial of the α-phase obtained from heating a crystal of the δ-phasedid not convert back to the yellow phase on cooling or it does so onlyvery slowly. This was likely due to the partial loss of formamidiniumforming a defective compound.

Thermal Analysis.

The thermal analysis results of the hybrid perovskites presented hereshow similar trends for Sn and Pb. The experiments were conducted undera nitrogen flow to avoid oxidation. Thermogravimetric analysis of thematerials showed that they decomposed before melting. Decompositionstarted at temperatures >300° C. and proceeded in one step. The massloss corresponds to loss of the organic cation and all the iodidecontent leaving a metallic residue.

The decomposition profiles seemed to be dependent on the syntheticconditions used to prepare the compounds. When the materials wereprepared in air the decomposition followed a different pathway thatincluded two distinct weight loss steps. In that case, the decompositionproceeded initially with a mass loss corresponding to “CH₃NH₃I” or“HC(NH₂)₂I”, possibly in the form of the free amine and HI, followed bya second step in mass loss corresponding to loss of HI molecules. Anexample of this is CH₃NH₃SnI₃, where the mass loss depended on thepreparative method employed. In that sense it seems unlikely that thefirst decomposition step is associated with loss of the organic cationalone. It is clearly seen that the mass loss is associated with thepurity of the material. For example, when the material was prepared bygrinding, the first step of decomposition displayed a much greater massloss as compared to the material prepared by high temperature solidstate or solution methods. A plausible explanation for this behavior isthe presence of Sn^(IV) species in the compounds. In that case, the massloss could be associated with the liberation of volatile SnI₄.Contributing to the weight loss is the liberation of CH₃NH₃I (in theform of CH₃NH₂ and HI) which is driven away due to the formation ofSn^(IV) species. It is worth pointing out that the decompositiontemperature for pure CH₃NH₃I is roughly 280° C., while the decompositionof partly oxidized CH₃NH₃SnI₃ (with weight loss) starts at ˜150° C.Further support for the presence of Sn^(IV) species, is provided by theabsence of a corresponding “two step” decomposition profile inCH₃NH₃PbI₃. In the former case this could be explained by the inabilityof Pb^(II) to oxidize to Pb^(IV) in an all iodide environment.

In the case of compounds 3 and 4 a similar behavior was observed withthe addition of a small mass loss associated with the evaporation offormamidine or one of its decomposition products. The case ofHC(NH₂)₂PbI₃ (4a) was significantly more complicated as there was somecontamination from phase (4b) which was probably responsible for thesmall exothermic mass loss observed at 270° C. A fully decolorized (palebrown) residue remained, indicating the complete loss of iodine from thesolid after the analysis.

Differential thermal analysis and differential scanning calorimetrystudies were performed on the materials, aiming to detect any possiblephase transitions that occur above room temperature. The only thermalevent that could be detected between room temperature and thedecomposition temperature was the evaporation of formamidine or aformamidine-related compound (endothermic) at 70° C. and an exothermicevent for 4a at 270° C. both of which were related to mass losses andwere therefore irreversible. The unexpected result came from CH₃NH₃PbI₃(3) where no reversible phase transition could be detected with DTA orDSC measurements. As stated above, CH₃NH₃PbI₃ did undergo a structuralphase transition going from tetragonal to pseudo-cubic symmetry between300 and 400K as determined by single-crystal diffraction studies. Thereason for the absence of any thermal signal for this transition maysuggest that the transition is of second order. The transition itselfdoes not involve chemical bond cleavage/formation; the only real changeis associated with lowering of symmetry. Subsequently, the change in theheat capacity of the material, C=Q/ΔT, is expected to be small andexplains the lack of signal in DTA- and DSC.

Optical Properties.

a) Electronic spectroscopy: The optical absorption of the materials wasmeasured using a diffuse reflectance UV-vis-NIR spectrometer. Allmaterials display a strong, abrupt absorption in the visible spectralregion (FIG. 22A and FIG. 22B). The absorption has been attributed tothe energy gap between the conduction and the valence bands, thusclassifying the materials as medium-bandgap semiconductors. The Sn andPb based materials display an optical bandgap between 1.2 and 1.6 eV, ingood agreement with the black color of the solids.

The band gaps of the Sn-based perovskites have mean values between1.2-1.35, with CH₃NH₃SnI₃ (1) having a consistently smaller band gapwith respect to HC(NH₂)₂SnI₃ (2) for every preparative method employed.We use the term “mean value” because as can be seen in the electronicabsorption spectra of FIGS. 22 (A) and (B) there is a variation of theband gap as a function of the preparative method. There is no obvioustrend on this variation besides the interesting fact that CH₃NH₃SnI₃ asprepared in an open tube (1.35 eV) and as prepared from solution or inthe sealed tube reaction (1.21 eV) show a very striking difference. Thisis despite the fact that their thermal behavior is very similar as istheir diffraction pattern. When compared with CsSnI₃, both 1 and 2 showa slight narrowing on the gap which is consistent with the lowering inthe symmetry of the [SnI₃]⁻ framework.

The Pb-based phases, however, have band gaps which were independent ofthe preparation method at 1.41 and 1.52 eV for HC(NH₂)₂PbI₃ (4a) andCH₃NH₃PbI₃ (3), respectively. As in the case of the Sn-phases, anarrowing of the band gap was observed for the Pb compounds uponsymmetry lowering, when compared to the room temperature absorption ofthe black perovskite form of CsPbI₃ obtained by a solid state reaction,before the material transitions to the yellow non-perovskitic phase(FIG. 22(C)). The absorption edge for both the Sn- and Pb-based phasesis sharp, suggesting a direct band separation.

b) Photoluminescence (PL) properties: The photoluminescence propertiesof specimens obtained from all four synthetic approaches (solution,grinding, open tube melting, sealed tube annealing) were measured usinga green light source at 532 nm and a specialized photoluminescencespectrometer. The solution samples showed no PL at all with theexception of HC(NH₂)₂PbI₃. FIGS. 23 (A) and (B) show typical emissionspectra of the hybrid perovskites prepared by grinding the precursorsand heating the samples at 350° C. for 20 sec.

Generally, the grinding method produced consistently highphotoluminescence. Annealing of the ground material also produced solidswith respectable luminescence, slightly weaker than the ground ones, butthe materials were highly homogeneous. However, it should be stated thatexhaustive annealing, i.e. >24 h quenched the PL emission (see below).When working under vacuum the materials obtained were pure andhomogeneous, though the PL-properties were noticeably weakened comparedto the materials prepared at ambient pressure. When these materials wereexposed to the atmosphere, they decomposed within a few minutes,displaying an extreme sensitivity. The solution-processed solids of bothSn and Pb perovskites were PL-inactive. This suggests that the presenceof defects such as Sn⁴⁺ species in the structure which are more likelyto be present in the samples made by solid state methods is probablyassociated with the photoluminescence.

As stated above, there is a significant variation of optical absorptionproperties of these materials that depends strongly on the preparationmethod. Therefore, it is not surprising that the emission propertiesdiffer to some extent. If PL comes from defects it should increase asthe symmetry is lowered via twinning, artificially creating defects, ordue to change of the band structure to generate PL-favorable conditions.The temperature dependence of the PL intensity/wavelength has beenconfirmed in CsSnI₃. When one of the other three methods (grinding, opentube melting, sealed tube annealing) was employed, the resultingpolycrystalline materials should have had many more defects. We supposethat this might be the source of photoluminescence enhancement. Afurther supporting indication of the defect-induced PL model is that theonly solution prepared material that showed some PL-activity wasHC(NH₂)₂PbI₃ which underwent a phase transformation from a perovskitestructure to a non-perovskite structure, incidentally forming a highlydefective perovskite phase during the transformation. Therefore, thedefects seem to play a crucial role in the expression of the PL, eitherwhen these are created during the synthesis or by means oftemperature-controlled structural changes. This is supported by theobserved trend which is: the cruder the synthesis the higher the PLintensity.

In FIGS. 23 (A) and (B), the PL properties of Sn and Pb hybrid materialsare presented along with the corresponding Cs phases. The magnitude ofthe emission is rather comparable with one another, except for CsPbI₃ inwhich the perovskite black phase, which is stable above ˜300° C., veryrapidly converts to the PL-non active yellow phase. The samples wereprepared from the melt in order for the data to be comparable. There isa small but real shift in the emission wavelength and each emissionwavelength matches quite well with the corresponding absorption energyedge.

Charge Transport Properties.

In order to further understand this behavior resistivity measurementswere performed on 1-3. Freshly prepared samples using the solutionmethod were employed using an isopropanol graphite suspension togenerate the electrical contacts. This was found to be a necessaryprecaution to avoid possible redox effects resulting from direct contactof the material with a metal surface. Single-crystals prepared from thesolution method (0.1-1 mm length) and cold-pressed pellets from theannealing method (1.67×1.67×5.00 mm³) were investigated. The sampleswere measured using both a PPMS 4-probe setup for low temperature dataand a conventional 4-probe method for room and high temperaturemeasurements.

Low-Temperature resistivity: Low temperature data were collected in the5-330K range for single-crystals of 1-3 using the PPMS instrumentationwhere a similar behavior for the three compounds was observed. Theinitial high resistivity values obtained at 5K gradually decreased asthe temperature increased and at ˜330K was 4 Ωcm for CH₃NH₃SnI₃, 34 Ωcmfor HC(NH₂)₂SnI₃ and 596 Ωcm for CH₃NH₃PbI₃). This trend in theresistivity vs. temperature was characteristic of semiconductors, unlikemetals which show the exact opposite trend. On cooling, resistivityincreased again down to 5K, slightly decreased in the case of 1 andslightly increased in the case of 2 and 3, showing a hystereticbehavior. This hysteresis comes as a consequence of the structural phasetransitions. In fact, the structural phase transitions were reflected asanomalies in the resistivity close to the transition temperature. Thus,compound 1 changed to the β-phase in the 200-220K region followed by onemore transition to the γ-phase around 130K. A resistivity increasearound 70K suggested more phase transitions were possible on loweringthe temperature. Similarly, for compounds 2 and 3 the β-phase wasreached at 260K and 320K, respectively, while the γ-phase was observedat 110K and 180K, respectively. The transition temperatures were in goodagreement with the transition observed in X-ray diffraction studiesdiscussed above. In the case of 3 where the α/β phase transition occursabove room temperature, resistivity data were also collected in the300-400K region using the setup described in the Experimental Section,revealing a sharp transition with an onset at 310K.

Arrhenius plots of the resistivity data showed that the phasetransitions have activation energies of two types related to bothelectronic and vibrational energy barriers. The high-energy electronicbarriers correspond to the jumping of electrons from the valence band tothe conduction band and indeed are in good (in 1) or fair (in 3)agreement with the energy gap obtained from the optical measurements. In2, however, no sharp electronic barrier showed up and instead theincrease in the resistivity progressed through broad, low activationenergy transitions which can be attributed to molecular motions of theorganic cation. Such low energy transitions can be seen in 1 as well.

Room Temperature and High-Temperature resistivity: Cold-pressedrectangular pellets (1.67×1.67×5.00 mm³) of (1-4) and/or elongatedrhombic dodecahedral single-crystal (0.5×0.5×0.5 mm³) of (1-3) were alsomeasured using the conventional 4-probe method. A remarkable behaviorcould be easily spotted differentiating CH₃NH₃PbI₃ from its otherperovskite counterparts. The difference lies in the strongly non-ohmiccharacteristics of 3 evident in the I-V curves of the measuredmaterials. On the other hand, CH₃NH₃SnI₃, HC(NH₂)SnI₃ and HC(NH₂)PbI₃display a nearly perfect ohmic behavior with a resistivity values of0.49, 11.8 and 3.4·10³ Ωcm at 300K following a declining trend withincreasing temperature.

CH₃NH₃PbI₃ was singled-out since it reproducibly displays a non-ohmicbehavior with a parabolic I-V dependence. The calculated resistivityvalue for a single-crystal of CH₃NH₃PbI₃, ignoring the non-linear partsof the plot, was 51 MΩ·cm at room temperature. Interestingly, theresistivity value varies slightly with the polarity of the appliedelectric field with a higher slope being obtained for negative polarity.The equilibrium value of the voltage for zero applied current (remnantpolarization) was roughly −10 mV, but in general displays a strongdependence on the history of the crystal. These characteristics stronglyimply a ferroelectric response (the point group 4 mm allows for such abehavior), however, as it will be explained below there is no sufficientevidence for this arising from charge transport measurements. Theresistivity graphs of compounds 1, 2 and 4 on the contrary, display anormal ohmic behavior

In terms of absolute resistivity there is a discrepancy between the low-and the high-temperature values. We rationalize this based on the I-Vcharacteristics of 1-3. At room temperature, 1, 2 and 4a, which adoptthe α-phase, displayed a nearly perfect ohmic behavior which resulted inresistivities in the 10⁰-10¹ Ωcm range for the Sn compounds and 10³ Ωcmfor the Pb compound. On the other hand, the β-phase of 3 displayed aremarkable non-ohmic behavior which was responsible for a hysteresisloop obtained in its I-V plot. The hysteretic behavior can probably beattributed to the reorientation of the permanent dipoles of CH₃NH₃ ⁺ onapplication of external electric field, in addition to the resistance ofthe inorganic {PbI₃}⁻ lattice. Thus, a capacitance/inductance factor wasintroduced to the charge transport which reflects the ability of thepermanent dipoles to align with the applied field while confined withinthe perovskite “cube”.

Thermopower: In order to assign the carrier type in the perovskitecompounds, we performed Seebeck thermopower measurements (Seebeckcoefficient) on both solution derived single-crystals of 1-3 andcold-pressed pellets of 1-4 fabricated from powders of the annealedsamples. The compound HC(NH₂)PbI₃ could only be measured in the latterform, since the pressure applied to form the pellets (˜10 GPa) wassufficient to stabilize the perovskite (black) form for several hours.The thermopower of the single crystals for compounds 2 and 3 in the300-400K range were negative and very large in magnitude indicating thatthe materials are n-type semiconductors with very low levels of electroncarriers. The Seebeck coefficients were −2700 and −6500 μV/K at 380K for2 and 3, respectively. These values are consistent with carrierconcentration of <10¹⁷ cm⁻³. For compound 3, the Seebeck value wassensitive to the phase transition, consistent with the overall change ofthe other transport properties.

In order to have a more complete picture of the transportcharacteristics, a complete collection of 1-4 and 5b (see below) wasprepared as cold-pressed from annealed powders obtained from thegrinding method. All samples prepared this way had nearly identicalgeometric dimensions which allowed for a direct comparison of theirelectron transport properties. Interestingly, with the exception of 4awhich is p-type, all samples displayed a n-type behavior. ThePb-containing samples invariably showed an extremely high Seebeckcoefficient in the order of few (for 5b) to several (for 3 and 4b) mV/Kindicative of very low carrier concentration. The Sn-based compounds(including a sample of similarly prepared CsSnI₃), on the other hand,showed significantly lower Seebeck coefficients, on the order of fewhundreds of μV/K, indicating a higher carrier concentration (still lowas an absolute value) with respect to the Pb analogues. The samplesstill behaved as semiconductors, as evidenced by resistivitymeasurements. It is important to point out that the thus measuredresistivity is not very precise since in that type of samples the grainboundaries are expected to significantly increase the resistivity.However, when the pressed pellets were made from not annealed samples, aspecial behavior was observed for the Sn compounds, which displayed aremarkable change in the sign of the thermopower behaving as p-typesemiconductors.

Hall-effect measurements: Hall effect measurements were also performedin the cold-pressed samples of annealed polycrystalline 1-3 and 5b inorder to estimate the carrier concentration of the compounds. Thespecimens were measured under a variable magnetic field of 0.5-1.25 Tunder a constant excitation of 10 μA for the less resistive samples (1,2, 5b) and 100 nA for compound 3. The results confirm the negative signof the carriers (n-type) for all the thus prepared compounds. Thecarrier concentrations were on the order of 10¹⁴ cm⁻³ for 1, 2 and 5b,while the respective carrier concentration for compound 3 was found tobe on the order of 10⁹ cm⁻³. For the former, data were in excellentagreement with the Seebeck measurement which suggest a very low carrierconcentration corresponding to nearly intrinsic semiconducting behavior.This argument is further corroborated by the fact that the specimens,except the p-type compound 4a, did not display any PL signal after themeasurements indicating depletion of the charge carriers. The depletionwas probably caused by the “exhaustive” thermal treatment during samplepreparation and several heating/cooling cycles during the measurements.Combining resistivity and Hall effect data (measured for the samespecimens), the electron mobility was calculated to be 2320 cm²/V·s forCH₃NH₃SnI₃, 103 cm²/V·s for HC(NH₂)₂SnI₃, 270 cm²/V·s forCH₃NH₃Sn_(0.5)Pb_(0.5)I₃ and 66 cm²/V·s for CH₃NH₃PbI₃.

Comparing our present experimental results with the establishedliterature data, we find that the evaluation on the charge transportproperties has been quite different than that previously anticipated.The resistivity for CH₃NH₃SnI₃ for example is higher by at least one ortwo orders of magnitude from previously reported samples, and this isdue to the differences in the synthetic approach, as discussed below.More importantly, we find that the overall evaluation of the Sncompounds as p-type “metals” needs to be revised since our resultsclearly indicate that CH₃NH₃SnI₃ and HC(NH₂)₂SnI₃ can also behave asn-type semiconductors. The reason for the ability of these compounds tobe p-type conductors (e.g. as in the case of CsSnI₃) is the uniqueproperty of the materials to spontaneously oxidize (through theSn²⁺/Sn⁴⁺ path) and thus to transition to a doped state with chargecarriers being holes through self-doping. The reason we were able toisolate the pure, very low charge carrier n-type semiconductors may befound in the use of a powerful reducing agent (i.e. H₃PO₂) which wasable to inhibit/reverse the self-doping process (i.e. suppress formationof Sn⁴⁺) and allows the isolation of the pure stoichiometric compounds.P-type conducting behavior was observed when the preparation of the Sncompounds was carried out through solid-state methods. The absence ofthe reducing agent rendered the Sn²⁺/Sn⁴⁺ conversion irreversible andtherefore the obtained solids were inherently doped. The fact that Snoxidation was responsible for the “metal-like” behavior in the solidstate is evident by direct comparison with the Pb-analogues (whichcannot undergo analogous Pb²⁺/Pb⁴⁺ doping) where the charge transportproperties are practically unaffected by the preparative route, althoughsome slight differences are still observed.

The most pronounced indication of this difference between the Sn and Pbcompounds comes from studying their mid-IR spectra. At this region, oneshould expect to observe only absorptions arising from the vibrationalmotion of the organic cations, which was, in fact, the case forcompounds 3 and 4a. However, in the case of Sn-based materials, theseabsorptions were masked by a broad absorption centered at ˜1000 cm⁻¹(100 meV). This was possibly an intra-band transition close the top ofthe valence band between full 5 s² orbitals (Sn²⁺) and (partially)vacant Ss⁰ orbitals (Sn⁴⁺). However, such a plasmon frequency requires acarrier concentration ˜10¹⁸-10¹⁹ cm⁻³ for a carrier effective mass ofm_(e)=1 which is not supported by charge transport measurements. ForCH₃NH₃SnI₃, where the largest and most intense plasmon frequency wasobserved, a huge electron mobility was also observed, which suggeststhat the effective mass of the CH₃NH₃SnI₃ compound should be very small.A similar behavior also occurred for compound 2, though it was much lessintense in magnitude, further supporting the mobility argument sinceHC(NH₂)₂SnI₃ has a significantly lower mobility than CH₃NH₃SnI₃.

However, it is still possible to “un-dope” the semiconductors to anearly carrier-depleted n-type state. This can be achieved withextensive thermal treatment at ˜200° C., at which temperature Sn⁺⁴dopants are eliminated in the form of volatile SnI₄. Excess of theorganic cations can also be partially removed at the annealingtemperature in the form of the free amine and HI gas. Therefore, it isfeasible to tune the doping levels within the Sn-based materials, andconsequently the overall charge transport characteristics, simply bychoosing the appropriate synthetic method to generate p- or n-typesemiconductors in the ASnI₃ family of compounds.

In addition, the above results illustrate that the phase transitionsdetermine the transport characteristics of the semiconducting perovskitecompounds. In their highest symmetry (α-phase), the perovskites displaythe maximum conductivity, which is a consequence of the maximum orbitaloverlap between the metal and the iodine (linear Sn—I—Sn bond). As thephase transitions progress at lower temperatures (β- and γ-phases) thechange in the M-I-M angles result in reduced orbital overlap, thushindering the charge transport through the inorganic framework. Thetransport properties of β-CH₃NH₃PbI₃, in particular, where acapacitor-like behavior was observed, may be rationalized by thereorientation of the CH₃NH₃ ⁺ dipoles on application of current throughthe material, which mask the properties of the inorganic [PbI₃]⁻framework.

CH₃NH₃Sn₁,PbI₃ solid solution studies: In the context of the aboveresults, we decided to study the properties of CH₃NH₃Sn_(1−x)Pb_(x)I₃solid solutions in order to explore the structure and property trends asa function of x. As shown above, these two phases follow the same phasetransition path, albeit the transitions occur at different temperatures.The materials were studied as a function of the preparation route asdescribed above.

The “quality” of the materials was clearly demonstrated here. It wasobvious that the ground samples contained impurities when compared withthe diffraction pattern of the simulated data from single-crystaldiffraction. The other three preparative methods (open tube melting,sealed tube annealing, solution) provided sufficiently pure materials.The weak Bragg reflections observed near the triplet of major peaks at24.7°, 28.6° and 32.1° 2θ angles was discussed earlier as an oxidationimpurity. For materials prepared from the melt, the peak appearedalready at the onset of the data collection. For samples obtained fromsolution synthesis and the annealed samples the minor reflections appearin the powder diffraction patterns only after prolonged exposure in airduring data collection. The cleanest pattern was obtained from theannealed samples, because of strong diffraction requiring a considerablyshorter collection time, so that substantial oxidation did not occur.Therefore, it was clear that these reflections were due to oxidation ofSn in the compounds since they did not appear in the diffractionpatterns of pure Pb and Pb-rich phases.

The evolution of the structure in this solid solution system wasconsistent, irrespective of the method used. The Sn-rich regions adoptedthe pseudocubic tetragonal P4 mm structure of the parent a-CH₃NH₃SnI₃compound up to x=0.5. For x>0.5 the structure changed to that of thetetragonal β-CH₃NH₃PbI₃, I4 cm.

The photoluminescence properties of the materials are strongly affectedby mixing the Sn and Pb sites (FIG. 24A and FIG. 24B). Again, materialsprepared from solution were not luminescent. The mixed-metal perovskitesprepared from the melt were also inactive, in contrast to the purephases prepared by the same method for x=0 and x=1. Intensephotoluminescence was produced when the materials were prepared by mildgrinding so that some unreacted solid remains (interface reaction). Thephotoluminescence intensity decreased as the mixture was thoroughlyground to a visually homogeneous black powder and the reaction wasalmost complete. The emission observed from this process corresponded tothe individual Sn and Pb centers showing two peaks at the respectiveemission wavelengths (FIG. 23A). However, when the material was placedin a sealed tube and annealed at 200° C., the formation of the solidsolution was complete and the Sn and Pb active centers were fully mixed.As a result, the initial emission peaks disappeared and a new emissionpeak occurred. This peak was red shifted and seemed to peak for x=0.25which emits at 1030 nm. This is an overall red shift of 70 nm withrespect to the end member CH₃NH₃SnI₃ (FIG. 24(B)). For x>0.25 the shiftis smaller as is the intensity of the peak. As a rule of thumb, theintensity seemed to be slightly decreased by annealing.

The red shift in the emission wavelength was reflected in the absorptionspectra which universally showed a shift to lower energies of absorption(FIG. 25A, FIG. 25B, FIG. 25C, and FIG. 25D). The largest band gap shiftoccurred in the materials obtained from solution, in the Pb-rich side ofthe solid solution reaching values of 1.1 eV. However, no emission couldbe detected in these specimens. The annealed materials showed anexcellent agreement between absorption energy and emission wavelength.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A near-infrared radiation source comprising: aradiation source; and a two-phase photoluminescent material comprising:a first phase of a compound having a formula selected from AMX₃, A₂MX₄,AM₂X₅or A₂MX₆, where the A is selected from elements from Group 1 of theperiodic table, the M is C, Ge, Sn or Pb, and X is selected fromelements from Group 17 of the periodic table; and a second phasecomprising a metal oxide of the formula (M′O_(y)′), to provide thetwo-phase material with the formula (AMX₃)_((1−x))(M′Oy′)_(x),(A₂MX₄)_((1−x))(M′O_(y)′)_(x),(AM₂X₅) _((1−x))(M′O_(y)′)_(x), or(A₂MX₆)_((1−x))(M′O_(y)′)_(x), where 0.1≤x≤0.6 and y′=1, 1.5 or 2, whereM′ is selected from elements of Groups 2-4 or elements of Groups 12-15of the periodic table; wherein the two-phase photoluminescent materialis configured such that it is irradiated by the radiation source whenthe radiation source is on; the two-phase photoluminescent materialbeing characterized in that it absorbs radiation from the radiationsource when the radiation source is on, whereby the material is inducedto emit photoluminescence in the wavelength range of 700 to 2000 nm. 2.The source of claim 1, wherein M and M′ are different elements.
 3. Thesource of claim 1, wherein the two-phase material has the formula:(AMX₃)_((1−x))(M′O_(1.5))_(x) and M′ is selected from B, Al, Ga, In, Sc,and Y.
 4. The source of claim 3, wherein the two-phase material has theformula (CsSnI₃)_((1−x))(AlO_(1.5))_(x).
 5. The source of claim 1,wherein the two-phase material has the formula(A₂MX₄)_((1−x))(M′O_(y)′)_(x), where y′ is 1 and M′ is selected from Si,Ge, Sn, and Pb.
 6. The source of claim 5, wherein the two-phase materialhas the formula (Cs₂SnI₄)_((1−x))(SnO)_(x).
 7. The source of claim 1,wherein the two-phase material has the formula (AMX₃)_((1−x))(M′O)_(x)and M′ is selected from Si, Ge, Sn, and Pb.
 8. The source of claim 7,wherein the two-phase material has the formula(CsSnI₃)_((1−x))(SnO)_(x).
 9. A near-infrared radiation sourcecomprising: a radiation source; and a two-phase photoluminescentmaterial having a formula selected from: Cs(SnI₃)_((1−x))(M′O)_(x),where M′ is Si, Ge, Sn, or Pb and 0.1≤x≤0.5; Cs(SnI₃)_((1−x))(M′O₂)_(x),where M′ is Si, Ge, Sn, or Pb and 0.1≤x≤0.5; andCs(SnI₃)_((1−x))(M′O_(1.5))_(x), where M′ is B, Al, Ga, In, Sc, or Y and0.1≤x≤0.5.
 10. The source of claim 9, wherein the two-phase material hasthe formula Cs(SnI₃)_((1−x))(SnO)_(x).
 11. The source of claim 9,wherein the two-phase photoluminescent material has the formulaCs(SnI₃)_((1−x))(M′O)_(x) or the formula Cs(SnI₃)_((1−x))(M′O₂)_(x) andM′ is selected from Si, Ge, and Pb.
 12. A detector, further comprising:a near-infrared radiation source comprising: a radiation source; and atwo-phase photoluminescent material comprising: a first phase of acompound having a formula selected from AMX₃, A₂MX₄, AM₂X₅ or A₂MX₆,where the A is selected from elements from Group 1 of the periodictable, the M is C, Ge, Sn or Pb, and X is selected from elements fromGroup 17 of the periodic table; and a second phase comprising a metaloxide of the formula (M′Oy′), to provide the two-phase material with theformula (AMX₃)_((1−x))(M′Oy′)_(x), (A₂MX₄)_((1−x))(M′O_(y)′)_(x),(AM₂X₅) _((1−x))(M′O_(y)′)_(x), or (A₂MX₆)_((1−x))(M′O_(y)′)_(x), where0.1≤x≤0.6 and y′=1, 1.5 or 2, where M′ is selected from elements ofGroups 2-4 or elements of Groups 12-15 of the periodic table; whereinthe two-phase photoluminescent material is configured such that it isirradiated by the radiation source when the radiation source is on; thetwo-phase photoluminescent material being characterized in that itabsorbs radiation from the radiation source when the radiation source ison; and a photoluminescence detector configured such that thephotoluminescence from the two-phase material impinges on thephotoluminescence detector when the near-infrared radiation source is inoperation.
 13. A near-infrared radiation source comprising: a radiationsource; and a two-phase photoluminescent material comprising: a firstphase of a compound having a formula selected from AMX₃, A₂MX₄, AM₂X₅ orA₂MX₆, where the A is selected from elements from Group 1 of theperiodic table, the M is C, Ge, Sn or Pb, and X is selected fromelements from Group 17 of the periodic table; and a second phasecomprising a metal halide of the formula A₂M′X₆ or A₃M′X₆, where M′ isselected from elements of Groups 2-4 or elements of Groups 12-15 of theperiodic table; wherein the two-phase photoluminescent material isconfigured such that it is irradiated by the radiation source when theradiation source is on; the two-phase photoluminescent material beingcharacterized in that it absorbs radiation from the radiation sourcewhen the radiation source is on, whereby the material is induced to emitphotoluminescence in the wavelength range of 700 to 2000 nm.
 14. Thesource of claim 13, wherein M and M′ are different elements.
 15. Thesource of claim 14, wherein the two-phase material has the formula(AMX₃)_((1−x))(AM′ _(0.5)F₃)_(x), where 0.1≤x≤0.5 and M′ is selectedfrom Si, Ge, Sn, Ti, Zr, and Hf.
 16. The source of claim 15, wherein thetwo-phase material has the formula (CsSnI₃)_((1−x))(CsSi_(0.5)F₃)_(x).17. The source of claim 14, wherein the two-phase material has theformula (Cs₂SnI₄)_((1−x))(CsM′_(0.5)F₃)_(x), where 0.1≤x≤0.5 and M′ isselected from Si, Ge, Sn.
 18. The source of claim 17, wherein thetwo-phase material has the formula (Cs₂SnI₄)_((1−x))(CsSi_(0.5)F₃)_(x).19. A detector comprising: a near-infrared radiation source comprising:a radiation source; and a two-phase photoluminescent materialcomprising: a first phase of a compound having a formula selected fromAMX₃, A₂MX₄, AM₂X₅ or A₂MX₆, where the A is selected from elements fromGroup 1 of the periodic table, the M is C, Ge, Sn or Pb, and X isselected from elements from Group 17 of the periodic table; and a secondphase comprising a metal halide of the formula A₂M′X₆ or A₃M′X₆, whereM′ is selected from elements of Groups 2-4 or elements of Groups 12-15of the periodic table; wherein the two-phase photoluminescent materialis configured such that it is irradiated by the radiation source whenthe radiation source is on; the two-phase photoluminescent materialbeing characterized in that it absorbs radiation from the radiationsource when the radiation source is on, whereby the material is inducedto emit photoluminescence in the wavelength range of 700 to 2000 nm; anda photoluminescence detector configured such that the photoluminescencefrom the two-phase material impinges on the photoluminescence detectorwhen the near-infrared radiation source is in operation.